Hide message for DBUnexpectedError exceptions

DBUnexpectedErrors are now treated like most other exceptions; we now
hide the error messages (which could contain sensitive information
such as IP addresses) unless $wgShowExceptionDetails is true.

In that case, the messages (and redacted backtraces) would be shown
even if $wgShowDBErrorBacktrace is false, though hiding them would
add complexity to the exception handler for little benefit.

Bug: 26811
Change-Id: I063c241975ce5b12a04abc21821ac67c716b3d5e