/help discards invalid command suffixes and inserts them in the middle of the help message

To reproduce, run /help experience addd. It will discard addd and use the longest valid prefix experience to request help entries for. In this case the user should at least be told that the entered command was invalid instead of silently ignoring part of it.

Alternative example: /help time set midnigh. This will behave the same as /help time set.

Update: as of 18w19a, the behavior is still the same, but invalid suffixes are now printed in the middle of help entries.