Improve handling for missing tasks.txt at startup
At some point in Python's history, the 'message' attribute on the IOError exception type was dropped.
We had been using IOError as a stand-in for the more accurate FileNotFoundError, and we don't really need to retrieve or print the original exception output, as far as I can tell.
This change updates the exception type, and emits a notification message, while retaining the existing behaviour that skips the error and continues on with starting gtimelog with an empty initial task list.
Closes #4 (closed).