Casting from an unregistered type does not throw.

Hello,

Issue description

I have this issue on pybind11 v2.5.0.

In the documentation, in object.rst, section Casting back and forth, the following is stated:

When conversion fails, both directions throw the exception cast_error.

But when trying to cast an unregistered custom type, it does not. Instead it seems to be setting an error (PyErr_Occurred returns a TypeError) and it returns a None object.

Reproducible example code

#include <pybind11/pybind11.h>
#include <pybind11/embed.h>
#include <cassert>

struct F {};

int main() {
  pybind11::scoped_interpreter interp;
  F f;
  try {
    auto obj = pybind11::cast(f);
  }
  catch(...) {
    return 0;
  }
  assert(((void)"cast did not throw an exception", false));
  return -1;
}

According to the documentation, we should not get an assertion failure with this code.