Make $wgShowExceptionDetails=false more feasible for production

Make $wgShowExceptionDetails=false more feasible for production

* Make the HTML error message prettier, with a nice red box and
  instructions to modify LocalSettings.php hidden in an HTML comment.
* Show the exception class name, since that's pretty safe.
* Show a random "log ID" to the user, and also send it to the exception
  log, to allow easier log correlation.
* Optionally send backtraces to the error log, enabled by default.

Change-Id: Ie92e46032b3d194c4217119567847a38a53be577