Gracefully handle r? of invalid user

Don't crash; instead post a comment informing people that that user
couldn't be assigned.

Also handle `r? @ghost` properly: treat it as clearing all assignees but
still do the other things that highfive normally does (e.g. setting
`S-waiting-on-review`).

highfive/newpr.py

@@ -105,16 +105,21 @@ def api_req(self, method, url, data=None, media_type=None):
 
     def set_assignee(self, assignee, owner, repo, issue, user, author, to_mention):
         try:
+            assignees = [] if assignee == 'ghost' else [assignee]
+
             self.api_req(
                 "PATCH", issue_url % (owner, repo, issue),
-                {"assignee": assignee}
+                {"assignees": assignees}
             )['body']
         except urllib.error.HTTPError as e:
             if e.code == 201:
                 pass
             else:
                 print(f"failed to assign {assignee} to {owner}/{repo}#{issue}")
-                raise e
+                print(f"error was: {e}")
+                print("posting error comment")
+                error_msg = f":stop_sign: @{assignee} could not be assigned"
+                self.post_comment(error_msg, owner, repo, issue)
 
         self.run_commands(to_mention, owner, repo, issue, user)