Detect missing Java when validating mustache template

[paulholden]

@stronk7 points to this line https://github.com/stronk7/moodle-local_ci/blob/master/mustache_lint/mustache_lint.php#L297

Example error in moodle-plugin-ci when Java is apparently not present

$ ./bin/moodle-plugin-ci mustache --moodle=/opt/moodle/master/src /opt/moodle/master/src/mod/forum
 RUN  Mustache Lint on mod_forum
/mnt/export/moodle/master/src/mod/forum/report/summary/templates/filter_dates.mustache - WARNING: Problem calling HTML validator - please report bug to integration team.

We can

1. Detect missing Java;
2. Switch to online