API: Check assert parameters earlier in the request

API: Check assert parameters earlier in the request

Specifically, check the assert and assertuser parameters before setting
up the action module, so errors in parsing the module's parameters due
to being logged out don't override the client's intended "am I logged
in?" check.

Note this means that assertion failures will no longer use custom module
output formatters. This seems like an acceptable tradeoff: on Wikimedia
sites in May 2018 there were no requests that would have been affected
by this change.

Bug: T197672
Change-Id: I02a71395d5ed9f445e57162f2136292825f8dbb5