Easier way to distinguish different UnsuccessfulCommandException errors

[samwilson]

Currently, UnsuccessfulCommandException is used when "The command did not produce any output" (i.e. the output file was empty or nonexistent, or there was nothing on stdout), as well as "The command could not be launched" (i.e. proc_open() failed).

Both of these situations occur within TesseractOCR::run(), so it would be useful to be able to catch them separately, e.g.:

try {
    $text = $tesseractOcr->run();
} catch ( NoOutputException $e ) {
    $text = '';
} catch ( ProcessCreationException $e ) {
    // Do something.
}

It seems that the state of "The command did not produce any output" is actually a bunch of different things — and it seems fine to handle these all together, but the trouble is that one of the states is when an image actually contains no text, and it seems like this shouldn't be an error. TesseractOCR::run() should be able to return an empty string, if that's the actual OCR output.