error_index_generator can fail with no error messages

[varkor]

See https://travis-ci.org/rust-lang/rust/jobs/472667448 for instance, in which the logs read:

[01:00:04] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools-bin/error_index_generator" "html" "/checkout/obj/build/x86_64-unknown-linux-gnu/doc/error-index.html"
[01:00:04] expected success, got: exit code: 101

This is completely unhelpful as it gives the user no idea what the problem is.

[GuillaumeGomez]

Interesting. All functions return a dyn Error which is checked at the top level in the main function. In case of errors, a panic! is emitted. So in theory you should see something in case of failure...

[Mark-Simulacrum]

Since we don't know what caused this and it's likely impossible/hard to reproduce I'm going to close this; if someone narrows down a reproduction then we can reopen to track a fix (though I suspect a PR would be relatively simple at that point, hopefully).

[gilescope]

In my case it was caused by #63016.