Make ./x.py errors more machine-grep-able

[mark-i-m]

We should systematically make all errors emitted by ./x.py more grep-able. This will make life easier for @TimNN and other bots.

[Mark-Simulacrum]

Could you be more specific? What do you mean by "errors emitted by x.py"? Most of these are test failures and compiler failures... which are already quite good, generally speaking, I'd imagine.

[mark-i-m]

Yes, I mostly mean compile and test failures -- anything that would fail a Travis build. Surprisingly, these are not as easy to find as one might expect, since TimNNBot often gives not-very-helpful snippets (e.g. too much or too little, such as #51335 (comment)).

Instead we should print something like "[ERROR START]" and "[ERROR END]" around the portions we want bots to pick up.

[Mark-Simulacrum]

The bot isn't based on start/stop lines iirc, so I'm not sure that's the right approach. @TimNN probably knows better than me, though.

[hellow554]

One way could be --output-format=json|xml|....
I mean, the effort to do this is quite high, but you have to decide if it's worth it.

[mark-i-m]

There doesn't seem to be high demand for this. Shall I close the issue?