backend-common: more informative error message when no url reader is matched

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/eighty-glasses-mate.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-common': patch
+---
+
+Use a more informative error message when URL reading isn't allowed due to no reader matching the target URL.

packages/backend-common/src/reading/UrlReaderPredicateMux.ts

@@ -52,7 +52,11 @@ export class UrlReaderPredicateMux implements UrlReader {
       }
     }
 
-    throw new NotAllowedError(`Reading from '${url}' is not allowed`);
+    throw new NotAllowedError(
+      `Reading from '${url}' is not allowed. ` +
+        `You may need to configure an integration for the target host, or add it ` +
+        `to the configured list of allowed hosts at 'backend.reading.allow'`,
+    );
   }
 
   async readUrl(