Hide server IP addresses from DB error pages

Error details are now omitted if both $wgShowHostnames and
$wgShowSQLErrors are false. WMF wikis have the former set to true;
users of those wikis can still include the details in bug reports.

Fixing this for DBConnectionError was more or less straightforward.
However, in the case of DBQueryError, this involved splitting the
error message into multiple smaller messages (killing a raw HTML
message in the process).

Note that this is an incomplete fix: information disclosure is,
for now, still possible from DBUnexpectedError exceptions (are
these even supposed to go into the exception log?) or exceptions
that occur from within the exception handler. Yet this is still
an improvement.

Bug: 26811
Change-Id: I1756b296d5e8d1d22511a3c3b58b5bb0dd025fec
Merge "Hide server IP addresses from DB error pages"