Relative path errors could be more explicit

See the exceptions in MNG-4824.

This was caused by a default relative path of '../pom.xml' which was not correct. Maven 2.x ignored it and went to the repository, but as specified in the compatibility notes Maven 3 fails. The exception should state that it was a non-resolvable parent because it is not in the repository and not found at the relative path of '../pom.xml'