Error handling

[burdges]

We add our own new error types, so we should add our own Error type that extends SynthesisError, but currently I just use ::std::io::Error with custom strings.

If we stick with zcash then all error types could ? into bellman::SynthesisError thanks to its IoError(::std::io::Error) variant, except..

We'd prefer ZEXE which presumable does error handling differently, especially since they support no_std.