Fix parallel worker crash on syntax error

[ilevkivskyi]

Fixes #21195

Fix is straightforward, and also makes tag reading convention similar to regular cache (i.e. caller reads the tag). @JukkaL when this is merged, I am going to rebase splitting PR to apply the same fix there.

Note that a cmdline (i.e. full) test is required for this, since this is not a crash from the point of view of the test harness, the workers simply dump tracebacks to stderr when they crash, while coordinator exits normally.

Btw, as I mentioned in original PR #20280, at some point I am going to redirect workers' stdout/stderr to a log file, similar to how we do it with the daemon. This would also fix this issue, but I prefer that workers always exit normally when possible, i.e. only genuinely unexpected conditions should crash the workers.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ilevkivskyi]

@JukkaL since the fix is trivial, I am not waiting for review to speed up process, please let me know still if you have any comments.