API: i18n for warnings and errors

API: i18n for warnings and errors

API warnings and error messages are currently hard-coded English
strings. This patch changes that.

With a few exceptions, this patch should be compatible with non-updated
extensions:
* The change to ApiBase::$messageMap will blow up anything trying to
  mess with it.
* The changes to the 'ApiCheckCanExecute' hook will cause a wrong
  (probably unparsed) error message to be emitted for extensions not
  already using an ApiMessage. Unless they're currently broken like
  Wikibase.

Bug: T37074
Bug: T47843
Depends-On: Ia2b66b57cd4eaddc30b3ffdd7b97d6ca3e02d898
Depends-On: I2e1bb975bb0045476c03ebe6cdec00259bae22ec
Depends-On: I53987bf87c48f6c00deec17a8e957d24fcc3eaa6
Depends-On: Ibf93a459eb62d30f7c70d20e91ec9faeb80d10ed
Depends-On: I3cf889811f44a15935e454dd42f081164d4a098c
Depends-On: Ieae527de86735ddcba34724730e8730fb277b99b
Depends-On: I535344c29d51521147c2a26c341dae38cec3e931
Change-Id: Iae0e2ce3bd42dd4776a9779664086119ac188412