release-23.1: c2c: fix rangefeed error propogration race

[blathers-crl]

Backport 1/1 commits from #102876 on behalf of @msbutler.

/cc @cockroachdb/release

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In the producer dist sql processor, rangefeed errors only propograte to the user if the error can be sent on a non-blocking channel. Because the channel was previously unbuffered, the dist sql processor would inadvertently swallow the error if the receiver was not actively waiting on the channel. This would then cause the sql processor to hang, as the underlying rangefeed would close after the ignored error message.

This patch buffers the errCh, guaranteeing that the first rangefeed error will be processed by the sql processor. If the rangefeed surfaces several errors while the buffered channel is full, these errors will be swallowed, which is fine, as the first error will always shut down the sql processor.

Fixes #102286

Release note: None

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Release justification: bug fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is