Add modifiers for error code and class (#576)

[kim]

Arguably the most naive / liberal way to allow for custom errors. There might be reasons to only allow a subset of the error codes / classes, but I can't make up my mind which ones that should be. If that's the case, perhaps a set of specialised constructors would be preferrable, otherwise I'd lean towards amending this to define a new constructor which takes the code and class directly.

Aside: I noticed that the ErrorCode enum is missing some possible values from the binding. I'll submit that separately.

[alexcrichton]

Thanks! Could these take &mut self though since I think that's more idiomatic?

[kim]

I think that's more idiomatic

I'm seeing "setters" used in a few different flavours across the ecosystem, so I can't tell, really :) mut Self -> Self, &mut self -> &mut Self, and &mut self -> () all yield very different ergonomics, and I chose the first because I figured that one would mostly want to construct custom errors, not actually modify them. So, what do you think of just new(class, code, message) or custom(...)?

[alexcrichton]

Adding new also sounds good to me! Typically with what I've seen with_x is self -> self while set_x is just &mut self -> (). Builder-style x is where &mut self -> &mut self comes into play

[alexcrichton]

Thanks!