Add better error message for AAPT file conflicts

Some resource directories may be the same even though
their names are different. For instance, the
"smallest width" qualifier was added in API 13,
so the resource directory "values-sw600dp" and
"values-sw600dp-v13" are the same and cause
a conflict. The error reports that this might be the
case.

Change-Id: Ia35f1d670edd48265b3a7fe3d55656128421f612