Add a flag to treat warnings as errors

[spiltcoffee]

$ yarn install
... <snip> ...
➤ YN0000: Done with warnings in 1s 337ms

Yet

$ echo $?
0

Would be nice if I could make yarn return non-zero with a flag, like yarn --treat-warnings-as-errors or some such thing. Use case would be in a CI pipeline, so that issues like YN0002 don't go unnoticed.