Fixed small running on YARN docs typo

The backslash is needed for multiline command

Author: Vlad <frolvlad@gmail.com>

Closes #1158 from frol/patch-1 and squashes the following commits:

e258044 [Vlad] Fixed small running on YARN docs typo
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