bugfix around this error: local variable 'failure_log' referenced before assignment

[hitokun-s]

HI. FIrst of all, thank you very much for creating such a wonderful library(and articles too).
I don't know you are accepting PR or not, anyways I send here.

When I executed:

./en/rte_en_mp_any.sh en/sample_en.txt en/semantic_templates_en_emnlp2015.yaml

I got error log like this:

ERROR:root:An error occurred: local variable 'failure_log' referenced before assignment

in en_results/sample_en.txt.***.err files.

(and as a result of this error, judgement become 'unknown' always.)
I investigated and found the cause in theorem.py, so fix it. Thanks.

[pasmargo]

Thank you for the PR and sorry for taking so long to answer. Let me try to merge. Thank you for your contribution!