[bugfix] improved authenticate post inbox error handling (#2803)

* improved PostInboxScheme() error handling / logging in case of failed auth

* dumbass kim. returning err instead of errWithCode...

* add checks for the slightly changed error handling in tests, add notes in codebase about the odd way of working

internal/api/activitypub/users/inboxpost_test.go

@@ -590,7 +590,7 @@ func (suite *InboxPostTestSuite) TestPostUnauthorized() {
 		requestingAccount,
 		targetAccount,
 		http.StatusUnauthorized,
-		`{"error":"Unauthorized: not authenticated"}`,
+		`{"error":"Unauthorized: http request wasn't signed or http signature was invalid: (verifier)"}`,
 		// Omit signature check middleware.
 	)
 }