When sub_test fails to parse an integer, write error message and exit cleanly.

This prevents start_test from also reporting that sub_test did not exit
cleanly. For example, previously on systems that don't clone symlinks
correctly, we would see the following two errors (both for the same issue):

```
[Error (sub_test): Invalid integer value in m-lsms.par-forall.numlocales (studies/lsms/shemmy)]
[Error running sub_test in /var/tmp/jenkins/chapel-ci/workspace/correctness-test-linux32-legacy-git/test/studies/lsms/shemmy (255)]
```

Now, only the first error will be reported. We _may_ want to consider making
clean_exit=True the default behavior for the Fatal() function in sub_test.