Error handling fixes

[kevinmehall]

Rationale for the IN transfer buffer size error:

This requirement has actually been in the documentation since v0.1.0 with this intention, just not enforced until now. It's a requirement for WinUSB RAW_IO, which nusb now always enables for IN endpoints on Windows because WinUSB has mediocre performance without it. Since Windows was returning an error in this case, I extended the check to all platforms for consistency and to eliminate the need to handle overflow errors.

You should think of an IN transfer of n bytes to be a request for n / max_packet_size packets, ending early after any short packet. It doesn't make sense to request less than a packet, and because the device, not the host, controls how large the packet is, the host needs to have space to receive it.

Users were incorrectly assuming it worked like Read::read() where you could use any size buffer and the remaining data would be kept for the next call, but what actually happens on Linux is that you get an overflow error and the remaining data is lost. nusb was returning confusing errors because the intention was that it shouldn't see an overflow error if the transfers are the expected size. This check makes it fail quickly if used like that. I'm adding EndpointRead / EndpointWrite for those who want the read() semantics.