[IronPython] Cryptic traceback when encoding name is incorrect

[Snaury]

# -*- coding: nonsense -*-
print "Testing..."

Traceback (most recent call last):
  File mscorlib, line unknown, in .ctor
  File mscorlib, line unknown, in .ctor
TypeError: Value cannot be null.
Parameter name: encoding

I think it should properly state that encoding name is invalid.