Factor out error behavior in dib-lint

Currently when dib-lint finds a problem it does something like:

 echo "ERROR: Problem found"
 rc=1

This is repetitive and error-prone since it's easy to forget to set
rc to actually fail the check.  This change makes those two steps
a single function call.

Change-Id: I40b5bf39348a69add1f955c49f310e3bda21be0e
Merge "Factor out error behavior in dib-lint"