[monodoc] Throw exception on unparseable URL

Instead of logging error with no useful context/content.
[monodoc] Return null from GetHelpXml if node not found

Instead of exploding the XML parser by trying to parse null.
[Mono.Data.Sqlite] Stop DataAdapter.Fill exploding when using Sqlite built w/o column metadata support

Fixes Bug 327 - Mono.Data.Sqlite doesn't work without SQLITE_ENABLE_COLUMN_METADATA

Essentially, if sqlite3_column_origin_name is not found, omit the metadata, instead of
exploding.  This is the fallback that's already used on MonoTouch.

A better fix would be load the column metadata from other sources, as proposed in
https://bugzilla.xamarin.com/show_bug.cgi?id=2128