Better error reporting for invalid site link page ids

This also makes ApiErrorReporter::dieMessage more focused and convenient.

Change-Id: I7cd98a2861947feb87dee55593dfa89f098dfdda
Replace invalid type hint

Change-Id: I80380e3e9d944b3be1a63baf21e192f97883b988
Improve no-such-sitelink message, use it for error.info

Change-Id: Ib0b1bdcf396facc5ff2f95d307306a035d23c0c8
Merge "Better error reporting for invalid site link page ids"
Merge "Replace invalid type hint"
Merge "Improve no-such-sitelink message, use it for error.info"