Show eval messages if debugging

This change checks the fabrik and joomla debug functions and issues eval
with or without the @ symbol which prevents php messages being displayed
depending on the debug setting.

This change to calc.php is intended as a proof of concept, with the same
code being rolled out to other uses of eval once this has been agreed.

Updated to use isDebug and make some other fixes.
Fix form save placeholder bug in repeat group
Improve error message on eval failure.
Increase width of calc editing box.
Fix calc_only_on_save.
Merge pull request #798 from Sophist-UK/enable_calc_error_messages

Show eval messages if debugging