Fix mvn detection

When mvn is not detected (not in executor's path), 'set -e' causes the
detection to terminate the script before the helpful error message can
be displayed.

Author: Matthew Farrellee <matt@redhat.com>

Closes #1181 from mattf/master-0 and squashes the following commits:

506549f [Matthew Farrellee] Fix mvn detection
(cherry picked from commit 853a2b951d4c7f6c6c37f53b465b3c7b77691b7c)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>