Fix status code for coverage API v3

* Change the status code of action 'reset' and 'stop' code to 204, and use
'wsgi.action' instead of action method.
* When stop coverage without started coverage will raise HTTPConflict
* Add decorator 'extensions.expected_errors'

Partially implements bp v3-api-extension-versioning

DocImpact

Change-Id: If9f873d85dbb10d9bfd1ae8a7122c3e3d94fc955
Merge "Fix status code for coverage API v3"