Improve WikiMap::getWikiReferenceFromWgConf()

If we don't have a valid canonical server and path to pass, there's no
point in returning a WikiReference that will fail in strange and unusal
ways.

This also documents that $wgServer/$wgCanonicalServer and $wgArticlePath
are required in SiteConfiguration.

Change-Id: Ib08011e9f1d0817a5d1bb165aba6b424785eaa6a