`path not exist` errors from `allowNotExist` check shows root relative paths instead of relative paths to working directory

[doriable]

Currently, when we do checks for allowNotExist for paths provided through the --path and --exclude-path flags, we only print out the root relative path, example:

https://github.com/bufbuild/buf/blob/94997f821e98f05f0cf63aa0901cadb9fe567d57/private/bufpkg/bufmodule/targeting_module.go#L155

We should be printing out the path relative to the user's working directory for more clarity in debugging.

[doriable]

Along the same lines, when we are returning errors to users around imports, we should indicate that in the error: https://github.com/bufbuild/buf-lint-action/issues/10

[bufdev]

I don't believe this path will necessarily be relative to a user's working directory - this could be within a tarball, git repo, etc.