Give a proper 'library not found' error instead of timing out

If there is a wrong path under one of the transformation scenarios in loris.conf, or/and if the binaries and libraries are not found, Loris just times out when requesting an image.

E.g.:
```
[[JP2]]
kdu_expand = '/wrong_path_to/kdu_expand'
```

This happens when using `mod_wsgi` and Apache under a Python3 virtual environment.