Exception Handling: Allow for exception fault handlers

The CLI provides the ability similar to 'finally'. Whereas 'finally' runs whether or not a try block exits normally, the 'fault' block only runs if the 'try' block exits abnormally through an exception. Both finally and fault run prior to other handlers.

http://www.microsoft.com/mspress/books/sampchap/5771.aspx

Implementation Notes:
Only one fault handler per try block should be allowed.
A fault handler can appear with a finally handler.
When composed with finally, we should probably enforce that fault should appear first and run before finally.

I recommend taking over the obsolete keyword failure for this use.
Proposed syntax:

try:
  raise Exception()
except:
  print "Caught"
failure:
  print "Exception thrown"
ensure:
  print "Completed"

This should print:

Caught
Exception thrown
Completed