Fix extensions agent follow API v3 rules

* Add decorator expected_errors
* Use AgentBuildExists instead of generic exception catching in create
* Use HTTPBadRequest instead of HTTPUnprocessableEntity
* Rename the request parameter 'para' to 'agent' for update

Partially implements bp v3-api-extension-versioning

DocImpact

Change-Id: I45203203b8396517ef94e604366183ce93810205
Merge "Fix extensions agent follow API v3 rules"