sql/opt/norm: propagate kv errors from cast folding

[ajwerner]

Swallowing KV errors here leads to incorrect results. Writes can be missed and serializability can be silently violated. This comes up in the context of the randomized schema change testing.

May deal with #85677
relates to #80764

Release note: None

[cockroach-teamcity]

This change is 

[ajwerner]

@mgartner how does this make you feel?

[ajwerner]

happy to put a TODO around this to say that we ought to mark the function as leakproof, or not, and propagate errors differently because of it.

[mgartner]

LGTM. I added the same logic to FoldCast and added TODOs. PTAL.

[mgartner]

Thanks for do this, btw! I had started working on this but I didn't love my solutions. I like this the best so far.

[ajwerner]

Thanks!

[ajwerner]

bors r+

[craig]

Build failed:

• Bazel Essential CI (Cockroach)

[ajwerner]

bors r+

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from b075b30 to blathers/backport-release-21.2-87614: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2.x failed. See errors above.

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