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:

buf/private/bufpkg/bufmodule/targeting_module.go

Line 155 in 94997f8

return nil, fmt.Errorf("path %q has no matching file in the module", potentialDirPath)

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: bufbuild/buf-lint-action#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.