update: recommend 'brew untap` if tap doesn't exist.

We removed a few old, deprecated repos recently and people with them
still tapped who get confused at the `brew update` error message.

Library/Homebrew/cmd/update.sh

@@ -510,9 +510,15 @@ EOS
         if ! git fetch --tags --force "${QUIET_ARGS[@]}" origin \
           "refs/heads/$UPSTREAM_BRANCH_DIR:refs/remotes/origin/$UPSTREAM_BRANCH_DIR"
         then
+          if [[ "$UPSTREAM_SHA_HTTP_CODE" = "404" ]]
+          then
+            TAP="${DIR#$HOMEBREW_LIBRARY/Taps/}"
+            echo "$TAP does not exist! Run 'brew untap $TAP'" >>"$update_failed_file"
+          else
             echo "Fetching $DIR failed!" >>"$update_failed_file"
           fi
         fi
+      fi
     ) &
   done