Cred retrieval failure info++ (#214)

* Refactor to if instead of ternary around GetCredentials

* Cred retrieval failure info++

If the problem is 'I don't have creds for that', the natural
follow-up is 'creds for what?', so let's answer that explicitly.

* fixup! Cred retrieval failure info++

Author: rainersigwald

Microsoft.NET.Build.Containers/AuthHandshakeMessageHandler.cs

@@ -2,12 +2,15 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Net;
 using System.Net.Http.Headers;
+using System.Runtime.CompilerServices;
 using System.Text;
 using System.Text.Json;
 using System.Text.RegularExpressions;
 
 using Valleysoft.DockerCredsProvider;
 
+using Microsoft.NET.Build.Containers.Credentials;
+
 namespace Microsoft.NET.Build.Containers;
 
 /// <summary>
@@ -89,9 +92,24 @@ private async Task<string> GetTokenAsync(Uri realm, string service, string scope
         string? credP = Environment.GetEnvironmentVariable("SDK_CONTAINER_REGISTRY_PWORD");
 
         // fetch creds for the host
-        DockerCredentials privateRepoCreds = (!string.IsNullOrEmpty(credU) && !string.IsNullOrEmpty(credP)) ?
-                                                                        new DockerCredentials(credU, credP) :
-                                                                        await CredsProvider.GetCredentialsAsync(realm.Host);
+        DockerCredentials? privateRepoCreds;
+
+        if (!string.IsNullOrEmpty(credU) && !string.IsNullOrEmpty(credP))
+        {
+            privateRepoCreds = new DockerCredentials(credU, credP);
+        }
+        else
+        {
+            try
+            {
+                privateRepoCreds = await CredsProvider.GetCredentialsAsync(realm.Host);
+            }
+            catch (Exception e)
+            {
+                throw new CredentialRetrievalException(realm.Host, e);
+            }
+        }
+
         // use those creds when calling the token provider
         var header = privateRepoCreds.Username == "<token>"
                         ? new AuthenticationHeaderValue("Bearer", privateRepoCreds.Password)

Microsoft.NET.Build.Containers/Credentials/CredentialRetrievalException.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace Microsoft.NET.Build.Containers.Credentials;
+
+internal class CredentialRetrievalException : Exception
+{
+	public CredentialRetrievalException(string registry, Exception innerException)
+		: base($"Failed retrieving credentials for \"{registry}\": {innerException.Message}", innerException)
+	{ }
+}