Create better error message when MAIN file does not exist

[ghost]

When the MAIN file referenced from a DC does not exist, DAPS prints a misleading error message:

# daps -d DC-suse-openstack-cloud-security validate
ERROR: Source files must reside in an "xml/" directory. "/path/to/repo/xml/MAIN.hos-imported.xml" is not a valid location.

Could you change the error message to something more easily understood, such as ERROR: MAIN file "/path/to/repo/xml/MAIN.hos-imported.xml" does not exist.?

[fsundermeyer]

Fixed with fbb3fd5