M101 - Mission Executive

@edwardhunter, @carueda Please Review
-Publish Mission Events
Merge branch 'master' of https://github.com/ooici/coi-services

Conflicts:
	ion/agents/mission_executive.py
M101 Mission Executive

-Publish MissionLoader events
M101 - fix failing unit test
M101 - Mission executive

-Add more mission executive event publishing
-Define a MissionErrorCode enum class for better readability
Merge pull request #2092 from Bobfrat/mission_exec

M101 - Mission executive