"improve" error handling in druid-shell

Error handling in druid shell is currently bad. We would like it to be good! This is a solvable problem!

Background

druid-shell started out as a windows-specific proof of concept. The existing error handling is left-over from that period. As druid-shell was adapted to be multi-backend, error handling was ignored, and now it's basically useless.

how it should work

Each platform should expose a platform-specific error type that mimics as closely as possible the system error type on that platform; so on macOS it should look like NSError, on windows it should be the system error code from GetLastError and the message returned by FormatMessage.

druid-shell's error module should have an Error enum that includes a Platform(_) variant for the platform specific error, and then normal variants for other conditions related to invalid inputs, io failure, etcetera.

We should return meaningful errors whenever possible.

Start with druid-shell's error module and map how platform errors are currently represented. Review the platform-specific behavior described for macOS NSError and Windows GetLastError/FormatMessage. Done means each platform exposes a meaningful error type and the error module includes Platform(_) plus variants for other conditions.