locale: fix incomplete fallback bundles

[BAMF0]

Fix an error where only the common strings would be used when falling back to embedded locales. This lead to an issue where the raw Fluent message keys were returned rather than actual error messages, such as wc-error-failed-to-print-result instead of "failed to print result".

Additionally, preserve partial resources instead of discarding everything when not all messages could be parsed.

Fixes #11854

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will not alter performance

✅ 331 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing BAMF0:main (cb82979) with main (df0701c)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[BAMF0]

Hmm the French tests seem to be failing unfortunately. They pass on my machine, I will have a look as to why.

[BAMF0]

Ah nevermind, the failing utility (hostid) does not have a french translation available under src/uu/hostid/locales, only en-US.ftl.

[BAMF0]

Superseded by #13257