redact exception traces and abstract getTrace

redact exception traces and abstract getTrace

* Partially reverts I0a9e92448 (rationale:
   http://www.gossamer-threads.com/lists/wiki/wikitech/401558)
  - wfDebugLog()'d exceptions are always unredacted
  - Other backtraces are redacted by replacing all argument values with class /
    type names.
* Adds a pair of static methods to MWExceptionHandler:
  - MWExceptionHandler::getRedactedTrace
	equivalent to Exception::getTrace, but replaces each argument value
	in the trace with its class or type name.
  - MWExceptionHandler::getRedactedTraceAsString
    equivalent to Exception::getTraceAsString, but with argument values
	likewise redacted.
* The rename of 'formatRedactedTrace' to 'getRedactedTraceAsString' is
  justified on two grounds:
  - 'formatRedactedTrace' didn't actually take a trace object (it took an
	exception).
  - 'getRedactedTraceAsString' maintains the symmetry with
    Exception::getTraceAsString.

Change-Id: I3d570a6385f96a606e1af53c50faa03b9ebacd38