twisted.spread.pb.CopiedFailure.raiseException can't work

[exarkun]

CopiedFailure always has a string for self.type. Any code that treats self.type as an exception class is going to encounter problems.

Previously this arose as a problem with `CopiedFailure.throwExceptionIntoGenerator`.

It is also a problem for CopiedFailure.raiseException, which simply does:

    raise self.type, self.value, self.tb

A string exception cannot be raised in the newest versions of Python, and even if it could be, it would be the wrong exception to raise.