Bug 1697909: 'custom_' unknown param not an exception in WS

If an LTI parameter is unknown and prefixed with 'custom_',
drop it from the data passed to the handler function.
Then if the site is in non-production mode,
also show this information.

behatnotneeded

Change-Id: I3fac19c27df2b9f072b8cdbcb8a2458c506c018e