Move all error definitions to `error` module
In `__init__`
- Remove imports from global scope
- Import all from `model` module
- Import all from `error` module
Refactor: `fetch_sources()` to take the URL as argument
Coding style: import definitions from `error` and `model`
Remove dependency to `webvtt-py` which was both too much and not enough
for our use case.
Implement a basic WebVTT to SRT converter according to ArteTV's usage of
WebVTT features.
