Shorten relative paths in warning texts

[quasilyte]

Given this warning text:

$GOPATH/src/github.com/go-critic/checkers/testdata/_integration/main_only/main.go:4:9: unslice: could simplify xs[:] to xs

...and granted that the checker was run from the main_only directory.
We can simplify the output to the:

./main_only/main.go:4:9: unslice: could simplify xs[:] to xs

This should only happen if -shorterErrLocation flag is set to true.