Exception Handler Logging and Improvements

This is a fork of #522746: Drupal Exception Wrapper Class so that at least a basic version can get into D7.

In the current version of core, only uncaught exceptions can reasonably provide info about the location where it was thrown easily. For caught exceptions that should still be logged so the site admin knows about it, logging that info is a hassle because of the way the functions are organized (they are clearly not meant to be called that way).

Also, the info logged could use some improvement as well.

Summary of changes:

* Added watchdog_exception to bootstrap.inc
* Moved _drupal_decode_exception from common.inc to bootstrap.inc (is now needed in the same scope as watchdog for processing trace data)
* Moved the watchdog call out of _drupal_log_error and into the handlers because Exceptions and errors now log slightly differently and this was preventing code re-use.
* Renamed _drupal_log_error to _drupal_display_error because it no longer logs, but still formats the error info for display to the screen / simpletests / etc.
* Simplified the usage of _drupal_get_last_caller by giving it a default backtrace if one isn't provided (debug_backtrace).
* Comment-only changes to simpletest module.

Net Effect:

* Caught exceptions can easily be logged if desired
* Added context for where the exception was caught, rather than only where it was thrown.
* The watchdog_exception caller can optionally add more info. Currently used to clearly label uncaught exceptions as such, see example below for other possible usage.

Example:

try {
  throw new Exception(t('test !one !two',array('!one'=>'a','!two'=>'b')));
} catch (Exception $e) {
	// Optionally log the Exception, if the site admin should know it happened.
	// Or, to add debug info while you're developing.
  watchdog_exception($e); // Normally this is all you need to provide.
  // If you want to provide extra info to aide in debugging, use this format.
	watchdog_exception(
	  $e,
		'Encountered during render of block %block_id',
	  array('%block_id' => '5'),
	  WATCHDOG_WARNING,
	  l('Account','User')
	);

}
return 'All is Well';