Allow passing detailed permission errors data to API

Using the new system introduced in
1c57794e371d74e1d88748de567a1c3358c3ad2e (see T47843).

This change allows Title::getUserPermissionsErrors() to include
MessageSpecifiers instead of string message keys in its return value.
This doesn't seem to have any bad effects, and should work seamlessly as
long as callers aren't trying to do anything stupid and just pass the
value to PermissionsError or OutputPage::showPermissionsErrorPage()
or wfMessage() or some such.

If the callers *are* trying something stupid, nothing worse than
duplicated or otherwise less-than-perfect error messages (in code
which tries to handle some message keys specially) should happen.
(I fixed wfMergeErrorArrays(), but who knows what else lurks in all
this code.) Any problems should only affect new-style errors using
MessageSpecifier, though.

Since MessageSpecifiers tend to be stringable, we probably won't get
fatals, but might get incorrect checks. Should we try to log this
happening somehow?

Goes with I42a0c5b0ea7e61088dd609b764dd7d1396c60cd5 in TitleBlacklist.

Bug: T115258
Change-Id: I1334ba21a2862973a9d8ff5be2c9bec06a82698b