MangaBaka: handle 404 cover image fetch gracefully

[gregoryn22]

Problem

When MangaBaka's CDN returns a 404 for a cover image URL that is present in the API response, fetchCover() throws an unhandled ClientRequestException. This exception propagates up through getSeriesMetadata() / matchSeriesMetadata(), causing the entire identify/match operation to fail even though the series metadata was fetched successfully.

This is a real-world reliability issue: MangaBaka sometimes includes cover URLs in API responses that no longer resolve (returning 404 + a PNG error body). The metadata itself is valid — only the artwork is missing.

Fix

Wrap the HTTP request in fetchCover() with a try/catch on ClientRequestException. On a 404 response:

• Log a warning with the series title and status code
• Return null (metadata is returned without a cover)

Any other HTTP error is re-thrown as before.

This mirrors the existing pattern in ComicVineMetadataProvider.getCover(), which already handles 404s this way.

Before / After

Before:

Fetch metadata ✅ → Fetch cover image → 404 → uncaught exception → entire identify fails ❌

After:

Fetch metadata ✅ → Fetch cover image → 404 → warn + return null → metadata returned without cover ✅

Notes

• Other providers (AniList, BookWalker, Bangumi, MAL) have similar unguarded image fetches, but that's out of scope for this PR. This change targets the immediate reported issue.
• getSeriesCover() is also protected by this fix, so a standalone cover refresh on a broken URL will also fail gracefully.