Add FetchMiddlewares.clarifyFailures and improve permission error handling

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

.changeset/app-defaults-clarify-failures.md

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+---
+'@backstage/app-defaults': patch
+---
+
+Added `FetchMiddlewares.clarifyFailures()` to the default fetch API middleware stack.

.changeset/clarify-fetch-failures.md

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+---
+'@backstage/core-app-api': minor
+---
+
+Added `FetchMiddlewares.clarifyFailures()` which replaces the uninformative "TypeError: Failed to fetch" with a message that includes the request method and URL.

.changeset/frontend-app-api-permission-error.md

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+---
+'@backstage/frontend-app-api': patch
+---
+
+Wrapped extension permission authorization in a try/catch to surface errors as `ForwardedError` with a clear message.