Python fastdispatch error message improvements

When overloaded C++ methods are called, the fastdispatch mode sometimes
optimises out the generation of the typecheck typemap code, resulting in
the 'Possible C/C++ prototypes' information not being in the error message.
Now when a TypeError is thrown, the exception message is modified to contain the
'Possible C/C++ prototypes'.

See issue #1293
6 files changed