cast error message should have the repr of the python type

[virtuald]

Currently, the error message is:

RuntimeError: Unable to cast Python instance to C++ type (#define PYBIND11_DETAILED_ERROR_MESSAGES or compile in debug mode for details)

That's not useful. We should be able to add (python type: <whatever>, C++ type: ???) to this error message to help users out even though we don't have the C++ type. If you're open to it, I can make a PR -- though we probably could bikeshed the error message contents.

[virtuald]

Also, we probably can add some kind of thread-local value that provides an optional context that could be inserted. This is particularly for things that are called by C++ functions from other C++ functions.