Pass error in humanize/canonicalize address to the contracts

[reuvenpo]

Currently, when an error happens on the Go side when parsing an address (both ways) the error is not returned to the contract, but instead the contract is halted.
We should add a Buffer field to the Go callbacks that will be filled in when an error happens, and then that error string should be passed into the wasm contract.
On the contract side, we need to expect it, and if we get it, convert it to an StdError variant.

[webmaster128]

We can report error strings since #443. However, in the import implementation we don't know which error types to report. Error strings are created by returning Ok(error_region_ptr), which is not happening in the current implementation.

pub fn do_canonicalize_address<A: Api, S: Storage, Q: Querier>(
    api: A,
    ctx: &mut Ctx,
    source_ptr: u32,
    destination_ptr: u32,
) -> VmResult<u32> {
    let source_data = read_region(ctx, source_ptr, MAX_LENGTH_HUMAN_ADDRESS)?;
    if source_data.is_empty() {
        return Ok(write_to_contract::<S, Q>(ctx, b"Input is empty")?);
    }

    let source_string = match String::from_utf8(source_data) {
        Ok(s) => s,
        Err(_) => return Ok(write_to_contract::<S, Q>(ctx, b"Input is not valid UTF-8")?),
    };
    let human: HumanAddr = source_string.into();
    match api.canonical_address(&human) {
        Ok(canon) => {
            write_region(ctx, destination_ptr, canon.as_slice())?;
            Ok(0)
        }
        // This check is unrelyable since we cannot tell by the error type if the error should be
        // reported to the contract or indicates a broken backend.
        // Err(FfiError::Other { error, .. }) => Ok(write_to_contract::<S, Q>(ctx, &error.as_bytes())?),
        Err(error) => Err(error.into()),
    }
}

pub fn do_humanize_address<A: Api>(
    api: A,
    ctx: &mut Ctx,
    source_ptr: u32,
    destination_ptr: u32,
) -> VmResult<u32> {
    let canonical = Binary(read_region(ctx, source_ptr, MAX_LENGTH_CANONICAL_ADDRESS)?);
    // TODO: how to report API errors back to the contract?
    let human = api.human_address(&CanonicalAddr(canonical))?;
    write_region(ctx, destination_ptr, human.as_str().as_bytes())?;
    Ok(0)
}

[webmaster128]

This will not be implemented as discussed in #445 (comment) and following comments.