Report errors in a C-usable format

[spindlebink]

As far as I can tell, there's no way to modify the format error messages are sent in, so WGPU errors get sent to their handler as a single line of unformatted object notation. It goes without saying that this makes debugging highly annoying.

For the Rust side of WGPU, this likely isn't a problem, since Rust has its own error handling and doesn't have to rely on a single C-side callback. But on the wgpu-native side, it'd be nice if errors could be reported in a format C can use--JSON at the very least would be usable.

Related to this issue: the WGPU log callback does not currently provide a way to provide a userdata pointer to its call, unlike the other callbacks, and doing so is pretty important for interoperability between non-Rust/non-C languages and the API.

[rajveermalviya]

error handling definitely needs an improvement, I am fine with serializing errors to json (but not sure how that would impact binary size). ideal solution would be to list enums of every possible errors and return them for type arg in the callback, but even then you will have to parse the message for meaningful conversion to other language structs.

otoh you could parse the current error message ^^

[rajveermalviya]

Related to this issue: the WGPU log callback does not currently provide a way to provide a userdata pointer to its call, unlike the other callbacks, and doing so is pretty important for interoperability between non-Rust/non-C languages and the API.

Opened #213 for this