Improve error message when a content resource or dependency cannot be found.

The message now mentions the cause of this error, which is either (1) the resource was missing, or (2) the resource was
present but not recognized as a supported format. The latter can occur, for example, due to an invalid bundle manifest or
blueprint XML file. This complements the new debug output introduced in
http://svn.apache.org/viewvc?view=revision&revision=1569792.

git-svn-id: https://svn.apache.org/repos/asf/aries/trunk@1571309 13f79535-47bb-0310-9956-ffa450edef68