Made the 'info' argument to SyntaxError optional, so phase-2 syntax

Made the 'info' argument to SyntaxError optional, so phase-2 syntax
errors are handled (these gave ``TypeError: not enough arguments'').

Also changed its __str__() to correct a typo (missing self.) and
return str(self.msg) to ensure the result is always string.

Also changed the default __str__ to simply return str(self.args).