New Rule Proposal: error for feed with frequent fetch errors

[owades]

Summary:

New rule: Confirm that >99% of requests to each RT feed in the past 24hr haven't resulted in a fetch error, as per the following best practice:

The server hosting GTFS Realtime data should be reliable and consistently return validly-formatted protobuf-encoded 
responses. Fewer than 1% of responses should be invalid (protobuf errors or fetching errors).

Severity: Error

Expected behavior:

If greater than 1% of requests to each RT feed in the past 24hr resulted in a fetch error, product an error.