Better exception handling

[N-Coder]

• handle (and log/mail) exceptions gracefully
• handle Telegram rate limit

[N-Coder]

Handle the following Telegram Errors when sending messages:

• RetryAfter: schedule retry after e.retry_after
• TimedOut: handle slow connection problems
• NetworkError: handle other connection problems
• ChatMigrated: the chat_id of a group has changed, use e.new_chat_id instead
• Unauthorized: remove update.message.chat_id from conversation list
• (BadRequest): handle malformed requests - read more below!
• (TelegramError): handle all other telegram related errors

[N-Coder]

Also handle exceptions from HTTP requests and the telegram background workers.

[N-Coder]

• [x] gracefully handle exceptions in the scheduler (see cb67c20a9b4069a413846cab4ba51271179ab623)
• [x] Telegram exception handling implemented in 2c59f20
• [ ] handle exceptions from HTTP requests
• [ ] handle exceptions from the telegram background workers