Fix some issues with the watcher

This patch fixes two bugs:
  - the state file is not saved because we use the method for checking
    for udpated data
  - in two places 'Error' was used instead of 'Exception', which breaks
    error handling

Additionally:
  - the unused 're' import has been removed
  - a variable named 'id' which collides with a builtin function has
    been renamed

Note that comparing the serialized forms might create false negatives
(due to the dicts being reordered) but that will just cause an extra
write of the file, which is sub-optimal but harmless.

Reviewed-by: ultrotter