Experiments watcher uses default paths.

If user doesn't provide config.neon for uploaded experiment, watcher
will use defaults paths for these sources:
  source sentences: source.txt
  reference sentences: reference.txt
Experiments watcher uses paths provided in config.
Watcher uses default if key is missing in config.
Watcher is complaining about missing resources.
Experiment's resources usage message improved.
Experiments watcher starts parsing resources.
Experiments watcher parses sentences from files.
Watcher checks that number of sentences matches.
EW aborts parsing of the experiment on error.
EW cleaned up.