Improve error messages from JSONCheckOff (#480)

Hopefully this will make the errors from `JSONCheckOff` and friends a bit more
consistent and comprehensible.

Author: richvdh

internal/match/json.go

@@ -92,7 +92,7 @@ func jsonCheckOffInternal(wantKey string, wantItems []interface{}, allowUnwanted
 	return func(body []byte) error {
 		res := gjson.GetBytes(body, wantKey)
 		if !res.Exists() {
-			return fmt.Errorf("missing key '%s'", wantKey)
+			return fmt.Errorf("JSONCheckOff: missing key '%s'", wantKey)
 		}
 		if !res.IsArray() && !res.IsObject() {
 			return fmt.Errorf("JSONCheckOff: key '%s' is not an array or object", wantKey)
@@ -106,7 +106,7 @@ func jsonCheckOffInternal(wantKey string, wantItems []interface{}, allowUnwanted
 			// convert it to something we can check off
 			item := mapper(itemRes)
 			if item == nil {
-				err = fmt.Errorf("JSONCheckOff: mapper function mapped %v to nil", itemRes.Raw)
+				err = fmt.Errorf("JSONCheckOff(%s): mapper function mapped %v to nil", wantKey, itemRes.Raw)
 				return false
 			}
 
@@ -119,7 +119,7 @@ func jsonCheckOffInternal(wantKey string, wantItems []interface{}, allowUnwanted
 				}
 			}
 			if !allowUnwantedItems && want == -1 {
-				err = fmt.Errorf("JSONCheckOff: unexpected item %s", item)
+				err = fmt.Errorf("JSONCheckOff(%s): unexpected item %v (mapped value %v)", wantKey, itemRes.Raw, item)
 				return false
 			}
 
@@ -132,6 +132,7 @@ func jsonCheckOffInternal(wantKey string, wantItems []interface{}, allowUnwanted
 			if fn != nil {
 				err = fn(item, val)
 				if err != nil {
+					err = fmt.Errorf("JSONCheckOff(%s): item %v failed checks: %w", wantKey, val, err)
 					return false
 				}
 			}
@@ -141,7 +142,7 @@ func jsonCheckOffInternal(wantKey string, wantItems []interface{}, allowUnwanted
 		// at this point we should have gone through all of wantItems.
 		// If we haven't then we expected to see some items but didn't.
 		if err == nil && len(wantItems) > 0 {
-			err = fmt.Errorf("JSONCheckOff: did not see items: %v", wantItems)
+			err = fmt.Errorf("JSONCheckOff(%s): did not see items: %v", wantKey, wantItems)
 		}
 
 		return err