Unknown command-line flags and/or non-existent files don't result in error code

This is particularly important for batch-mode, but I think it applies universally. Unknown flags and/or missing files should cause alspro to exit with an appropriate error code (2+).

Example Misbehavior:

```
$ ./alspro -foo -b missing.pro -g true && echo 'no error code!'
Error: File does not exist: -foo.
Error: File does not exist: missing.pro.

no error code!
```