Add `--fail-on-warnings` flag

[CarrotManMatt]

prek will print a warning message to stderr if there is a problem with configuration file, like unexpected keys. This warning is output, but execution continues if the problem is not fatal and leaves unambiguous configuration.

It would be great for my use case to be able to exit with a non-zero exit code whenever one of these warnings is about to be produced, even if all hooks passed. Of course this should not be default behaviour, so I suggest the use of the flag --faiil-on-warnings (with the corresponding environment variables).

Initially I thought this could be rolled into one of the report levels suggested in #1777, but seeing that the intention is for the report status levels to not affect the exit code, I split it into a new idea.

[CarrotManMatt]

Apologies if I have missed something and this is already possible.

My current workaround is the following:

prek run --color never | tee /tmp/prek.log
if grep -q '^warning: ' /tmp/prek.log; then exit 1; fi