release-24.3: catalog/descs: avoid log.Fatal for invalid descriptor error

[blathers-crl]

Backport 1/1 commits from #136978 on behalf of @rafiss.

/cc @cockroachdb/release

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Rather than crashing the whole process, we can use an assertion error for this. The log.Fatal was initially added in 5d205ed, in a time far before we were careful about avoiding node crashes.

fixes #136962
fixes #136961

Release note: None

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Release justification: low risk change to error

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[fqazi]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @spilchen)