better error messages from subprocess executions

If you get an exception from trying to execute a subprocess (commonly because
the program isn't installed or isn't on your PATH) then include the name of the
program and the arguments to it in the resulting stack trace.
Merge branch 'master' into 6_better_error_messages_from_subprocess_execution

Conflicts:
	code/language_modelling/SRILMServerPipe.py