Specify the utf-8 charset on error messages.

Although the contents of error messages are English text,
and thus it's irrelevant that it gets interpreted as
iso-8859-1 instead of utf-8, $wgSitename is printed by
DBConnectionError::searchForm(), and that can contain utf-8.

Cached pages may also be in utf-8.

Change-Id: I81c02493f0824cb010c80f4356f338787d78df6b