* add get mute timing by name to MuteTimingService
* update get mute timing request handler to use the service method
* replace validation, uniqueness and used errors with errutils
* update mute timing methods return errutil responses
* use the term "time interval" in errors bevause mute timings are deprecated in Alertmanager and will be replaced by time intervals in the future.
* update create and update methods to return struct instead of pointer
Yuri Tseretyan
4479e7218d