Added a function clause to edts_eunit:format_error to handle exits in a ...

I got the following error in edts_resource_eunit:resource_exists/2 attempting to run a test suite:
```
{badmatch,{error,not_found}}
```

The cause was a function-clause in edits_eunit:format_error/1 which prevented edts_resource_eunit form obtaining the eunit test results. The reason for the function-clause was because my test suite failed with:
```
{'EXIT',{aborted,{no_exists, some_mnesia_table}}}
```
which didnt match the expected {error, Reason}. The fix could be to add a catch all clause to format_error. My pull request just adds code for the exit case.