Fix identifier create to error on reusing an AID alias

[pfeairheller]

In IdentifierCollectionEnd.on_post method a check to see if name already needed at the beginning of the method and a 400 Bad Request exception should be raised on a second attempt to reuse a name (aid alias).

[rodolfomiranda]

I corrected that issue in #78.

[lenkan]

This is already implemented

keria/src/keria/app/aiding.py

Lines 471 to 474 in cfc55d4

	if agent.hby.habByName(name) is not None:
	raise falcon.HTTPBadRequest(
	title=f"AID with name {name} already incepted"
	)

and can be closed I think.

cc @2byrds