Add Error Message Handling for Test by Generation

[rosypen]

Test by Generation makes calls to ACE or LKB which can fail for various reasons (maybe the grammar cant be compiled, maybe no sentences could be parsed/generated, maybe there were memory issues, etc.)

There should be some graceful error message handling on the test by Generation page to report what went wrong and possibly some instructions on what to do. In particular, for test by generation using test sentences, when the test sentences were not able to be parsed it should at least report that an attempt was made but it failed at parsing for example.

The code itself can also lend itself to some bugs as well because there are expectations about the formatting of many things like the choices file or the predication schemes so if any of those change there will likely be an error.

It's a good idea to just put some kind of try-except around potentially problematic areas.