Make error types serializable

[bddap]

This pr derives serialize and deserialize for public error types. This is needed for structured errors in networked apis.

Note 1

core::ser::Error required some extra attention as it includes an io::ErrorKind in it's definition. Removing io::ErrorKind would make obviate the need for manual serialization, but io::ErrorKind may be needed elsewhere.

The current solution works fine, but adds some boilerplate.

Note 2

This pr is blocked until secp is updated to 0.7.5. The code in this pr will not compile until 0.7.5 is published.

Once secp is updated, this pr will be ready to merge.

[antiochp]

What would one of these errors look like serialized into json? Can you paste an example here (even a representative example).

I'm wondering how "rust specific" this is going to feel in terms of error handling on the other end.

[bddap]

Here is an example of a serialized grin_keychain::extkey_bip32::Error

{"InvalidChildNumber":{"Normal":{"index":10}}}

and here's a libwallet::ErrorKind

{"Keychain":{"KeyDerivation":{"InvalidChildNumber":{"Normal":{"index":10}}}}}

code

use grin_keychain;
use grin_libwallet;
use serde_json;
use std::io::stdout;

let example_err = grin_libwallet::ErrorKind::Keychain(grin_keychain::Error::KeyDerivation(
    grin_keychain::extkey_bip32::Error::InvalidChildNumber(
        grin_keychain::extkey_bip32::ChildNumber::Normal { index: 10 },
    ),
));
serde_json::to_writer(stdout(), &example_err).unwrap();

The serialized error is rusty, but definitely human readable, also parsable by non-rust clients.

Some nice things about sending serialized errors:

• Network apis become isomorphic to native ones. For every method API::m(args..) -> R, a method NetworkAPI::m(args..) -> Future<Item=R, Error=NetworkError> can be generated.
• Rust (and possibly non-rust) clients can be generated automatically.
• Implementors of non-rust clients can use just the type definitions from rustdoc to parse structured errors.

[bddap]

Another thought: non-rust clients may simply ignore as much error structure as they want. They can even store errors as generic json blobs.

[bddap]

Update:

• It's been a while and Bump version and upload to crates.io rust-secp256k1-zkp#45 is not resolved so I patched the secp dependency to pull from github.com/mimblewimble/rust-secp256k1-zkp.
• Rebased onto mimblewimble:milestone/1.1.0
• Marked as not WIP