<!--A few plain bullets saying what changed and why, for a reviewer
skimming it - not a wall of text, not a restatement of the diff, not
generated boilerplate.-->
#### Description
Webhook notification channels with a bearer token authorisation work in
v1 with all three spellings `Bearer/bearer/BEARER`, but v2 API was not
accepting anything other than `Bearer`. This PR changes the conversion
from receiver -> gettable flow.
Also, error messages are made better in 2 places.