x/tools/gopls: set non-zero exit code on check command when there are errors

[silverwind]

Proposal Details

gopls check is useful to lint code, but it currently always sets exit code 0 which is not ideal when one wants to run it for example on a CI:

$ gopls check internal/cmd/check.go; echo $?
main.go:10:24: unused parameter: file
0

I suggest to either always set exit code to non-zero when there are errors, or introduce a new --exit-code option to do the same.