installer: shows status message on DB connection error

While installing MediaWiki with a sqlite backend and missing the sqlite
extension, I have received the very helpful message:

  DatabaseInstaller::setupSchemaVars: unexpected DB connection error

Which to me is as helpful as:

  error

Since the database connection returns a Status object and that it got a
message attached, we might as well pass the error message when raising
the exception.  We would end up with an error like:

 DB connection error: could not find driver.
 Check the data directory and database name below and try again.

That let me instantly facepalm and install the sqlite PHP extension
instead of wasting time figuring out what is happening.

Change-Id: Ife8e97242e1cfbdfbb572ad50cf8e4a7ad5b9dfc