[TASK] Additional check for unavailable python

In case $TYPO3_CONF_VARS['BE']['disable_exec_function'] is set to '1',
exec() is completely disabled and python (and unzip) will never be
"found". A friendly check and hint will help administrators to spot the
problem.

Change-Id: I819215b3d35a3eaf9940e98cfa54904c83984db7
Reviewed-on: https://review.typo3.org/25088
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers