candidates2corpus.py: verify that splitters exist

[achimr]

If the splitters specified in the command line do not exist this just fails silently and the script still runs through the entire corpus download.

[achimr]

In https://github.com/ModernMT/DataCollection/blob/65e4700ed46910c1e7e7e7e1bf5a541511650a64/baseline/external_processor.py#L19
subprocess.Popen should throw an OsError exception if the splitter command doesn't exist. Curious why it doesn't in this case.

[achimr]

In the constructor above self.cmd is an array, as the ExternalTextProcessor object is constructed like this:
https://github.com/ModernMT/DataCollection/blob/65e4700ed46910c1e7e7e7e1bf5a541511650a64/baseline/external_processor.py#L82

[achimr]

Well, the splitter is executed in a thread
https://github.com/ModernMT/DataCollection/blob/65e4700ed46910c1e7e7e7e1bf5a541511650a64/baseline/external_processor.py#L31
Can't really catch exceptions in threads https://stackoverflow.com/questions/2829329/catch-a-threads-exception-in-the-caller-thread-in-python
So need to check earlier if command exists.

[achimr]

self.cmd in the Popen call above can be a command string or an array. Only if it is an array one can check if the first parameter is actually an available command (either checking the full path and/or searching for it in PATH). This would only partially fix the problem. Not fixing this for now.
Needs to be looked into with larger context how to call the splitters from the script https://stackoverflow.com/questions/2629680/deciding-among-subprocess-multiprocessing-and-thread-in-python