Audit the priority levels for NewGtfsErrorType

[landonreed]

Many of the NewGtfsErrorType priority levels appear to be mismatched with reality. Given recent work in Data Tools server to expose issue priority for display in the UI, these priority levels need an audit in order to verify their accuracy. Perhaps a set of criteria should be developed to help equate priority to a set of practical effects (e.g., high priority assigned if GTFS feed breaks during OTP build, medium priority if trip or other entity is dropped, low priority if no effect in third party app).