BUG: Report error if --python-script associated with non-existent file

[jcfr]

This commit ensures an exception is raised when the python script does not
exist. When associated with the --exit-after-startup option, a non-zero status
code is now returned when Slicer exit.

[jcfr]

Cc: @lassoan @ihnorton

[lassoan]

Would it be possible to throw the error from inside this->corePythonManager()->executeFile()?

Execution can go wrong in many ways and it is odd that one particular error is reported with an external mechanism.

At least we should let executeFile decide if there is an error (file does not exist, not readable, etc) instead of just checking outside if the file exists. This would be also more robust, when script is defined by relative path (executeFile might be able to find it, while QFile::exists cannot).

[jcfr]

Thanks for the review

Would it be possible to throw the error from inside this->corePythonManager()->executeFile()

Definitively, that makes sense.

For reference:

https://github.com/commontk/CTK/blob/6c0eddaaf8b306c5cf553e8421bca7a5640e380a/Libs/Scripting/Python/Core/ctkAbstractPythonManager.cpp#L303-L333