Fix extension server_meta follow API v3 rules

* add decorator expected_errors
* change 'meta' to 'metadata' for show and update that is
consistent with other actions
* use is_valid_body instead of catching AttributeError and ValueError,
because AttributeError and ValueError is too generic.

Partially implements bp v3-api-extension-versioning

DocImpact

Change-Id: Icdb353b077620c1ba690f3679dea6a8956847d36
Merge "Fix extension server_meta follow API v3 rules"