Publish file-map validation errors now name the actual base directory instead of a fixed phrase, and gained a 'pass a relative path' hint

When publishing files (for example, to an artifact or site) fails because a file falls outside the allowed folder, the error message used to always say "the working directory" no matter what folder was actually being used. Now it names the actual base folder the publish was rooted at. These errors also gained a new hint suggesting the reader pass a relative path instead.