Add update interval exponential backoff on source collector failure

[pederhan]

This PR implements more sensible failure strategy defaults for source collector errors. The following is changed:

1. The error tolerance stuff is now opt-in by default.
2. The default strategy for source collector errors is now to apply a delay using exponential backoff to the source's update interval on failure.
3. exit_on_error is now False by default, and must be opted in to. The default behavior is now to use the backoff strategy outlined above. This is technically a breaking change, as failing sources would terminate the application prior to this PR.

All in all, this PR brings in more sensible error handling defaults for source collectors and makes the application more likely to recover from intermittent errors.