Refactor duplicate error handling into fetchErrorMessage helper (#7644)

## What Changed
Extracts duplicate error handling logic for 404/500 status codes in
`VerifyThenLoadAPIContent` into a reusable `fetchErrorMessage` helper
function.

Addresses review feedback from #7642.

## Type
- [ ] ✨ Feature
- [ ] 🐛 Bug fix
- [x] ♻️ Refactor
- [ ] 🏗️ Build/Infrastructure
- [ ] 🔒 Security

## Testing
No functional change. The helper function encapsulates the same AJAX
call pattern previously duplicated in both 404 and 500 handlers.

Before (repeated in both handlers):
```javascript
404: function () {
    try {
        $.ajax({
            method: "GET",
            url: url,
            async: false,
            dataType: "json",
            success: function (data) {
                var msg = data && data.message ? data.message : error;
                window.CRM.DisplayErrorMessage(url, { message: msg });
            },
            error: function () {
                window.CRM.DisplayErrorMessage(url, { message: error });
            },
        });
    } catch (e) {
        window.CRM.DisplayErrorMessage(url, { message: error });
    }
}
```

After:
```javascript
404: function () {
    fetchErrorMessage(url, error, function (msg) {
        window.CRM.DisplayErrorMessage(url, { message: msg });
    });
}
```

## Screenshots
N/A - No UI changes

## Security Check
- [ ] Introduces new input validation
- [ ] Modifies authentication/authorization
- [ ] Affects data privacy/GDPR

### Code Quality
- [x] Database: Propel ORM only, no raw SQL
- [x] No deprecated attributes (align, valign, nowrap, border,
cellpadding, cellspacing, bgcolor)
- [x] Bootstrap CSS classes used
- [x] All CSS bundled via webpack

## Pre-Merge
- [x] Tested locally
- [x] No new warnings
- [x] Build passes
- [x] Backward compatible (or migration documented)

<!-- START COPILOT CODING AGENT TIPS -->
---

✨ Let Copilot coding agent [set things up for
you](https://github.com/ChurchCRM/CRM/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot)
— coding agent works faster and does higher quality work when set up for
your repo.

Author: DawoudIO

src/skin/js/CRMJSOM.js

@@ -53,6 +53,28 @@ window.CRM.VerifyThenLoadAPIContent = function (url) {
     var error = i18next.t(
         "There was a problem retrieving the requested object",
     );
+
+    // Helper function to fetch error message from JSON response
+    function fetchErrorMessage(targetUrl, fallbackError, callback) {
+        try {
+            $.ajax({
+                method: "GET",
+                url: targetUrl,
+                async: false,
+                dataType: "json",
+                success: function (data) {
+                    var msg = data && data.message ? data.message : fallbackError;
+                    callback(msg);
+                },
+                error: function () {
+                    callback(fallbackError);
+                },
+            });
+        } catch (e) {
+            callback(fallbackError);
+        }
+    }
+
     $.ajax({
         method: "HEAD",
         url: url,
@@ -62,44 +84,14 @@ window.CRM.VerifyThenLoadAPIContent = function (url) {
                 window.open(url);
             },
             404: function () {
-                // Try to fetch the body to get a more helpful message (the route may return JSON)
-                try {
-                    $.ajax({
-                        method: "GET",
-                        url: url,
-                        async: false,
-                        dataType: "json",
-                        success: function (data) {
-                            var msg = data && data.message ? data.message : error;
-                            window.CRM.DisplayErrorMessage(url, { message: msg });
-                        },
-                        error: function () {
-                            window.CRM.DisplayErrorMessage(url, { message: error });
-                        },
-                    });
-                } catch (e) {
-                    window.CRM.DisplayErrorMessage(url, { message: error });
-                }
+                fetchErrorMessage(url, error, function (msg) {
+                    window.CRM.DisplayErrorMessage(url, { message: msg });
+                });
             },
             500: function () {
-                // For 500, try to fetch JSON body for a clearer message
-                try {
-                    $.ajax({
-                        method: "GET",
-                        url: url,
-                        async: false,
-                        dataType: "json",
-                        success: function (data) {
-                            var msg = data && data.message ? data.message : error;
-                            window.CRM.DisplayErrorMessage(url, { message: msg });
-                        },
-                        error: function () {
-                            window.CRM.DisplayErrorMessage(url, { message: error });
-                        },
-                    });
-                } catch (e) {
-                    window.CRM.DisplayErrorMessage(url, { message: error });
-                }
+                fetchErrorMessage(url, error, function (msg) {
+                    window.CRM.DisplayErrorMessage(url, { message: msg });
+                });
             },
         },
     });