Improve the error struct

[roblabla]

Right now, our errors are wrapped into a rust newtype: https://github.com/roblabla/megaton-hammer/blob/master/megaton-hammer/src/lib.rs#L56

There are several additions we should do to it:

1. This should probably be #[repr(transparent)]
2. By using https://doc.rust-lang.org/core/nonzero/struct.NonZero.html and a constructor, we could make errors look like Option<Error>, while still being internally represented as a single u32, retaining binary compatibility (and thus being truly zero-overhead). Then, we could have all the svcs return an Option<Error>, giving us the ability to pattern match over its result.
   • TODO: Check if Result<(), Error> would be represented as a single u32 as well. If it is, then that's even better!
3. We should have an enum of Modules (see http://switchbrew.org/index.php?title=Error_codes#Modules), which we can then stringify. In fact, maybe we should bitfieldify the error code ? Using something similar to https://github.com/roblabla/megaton-hammer/blob/master/megaton-hammer/src/ipc/mod.rs#L25