Fix extension hosts follow API v3 rules

* Add decorator expected_errors
* Rename parameter 'updates' to 'host' for update
* Add input validation
* Tidy up exception catching

Partially implements bp v3-api-extension-versioning

DocImpact

Change-Id: Ic7858460a1ebaf2e1981792b837e5858c7e9cccf
Merge "Fix extension hosts follow API v3 rules"