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
Merge branch 'master' of https://github.com/lamber/gc
raise the java heap size limit

Generating pos_ngram_models with the extra external corpus is taking many GB of
RAM. (On an EC2 instance.)
error diagnostics, bz2, name files with errprob
make this shell script executable
Skip the essay randomiser test for now, since we are changing the proportion.  TODO: parameterise the proportion and use original proportion for testing.
Downgrading debugging the srilm server to an option.