[v2] Use RpcResponse<T> instead of exception-driven development

[skynetcap]

Discuss:

For Version 2, I'd like to remove the required try-catch'ing of RpcException. I've received feedback that it's a bad pattern to catch exceptions as a requirement. I believe changing the method signatures on RpcApi would break all clients, thus a major version (v2) would have these changes. I'm strongly against breaking clients, but v2 is needed. I could be wrong on any of these so please opine.

Example:

Current: (requires a try-catch block which is deal-breaking for some)

try {
    long balance = client.getApi().getBalance(...);
} catch (RpcException ex) {
    log.error(ex);
}

Future: (does not require exceptions)

RpcResponse<Long> response = client.getApi().getBalance();
if (response.isSuccessful()) {
    // happy path
    long balance = response.getValue();
} else {
    // handle/log error gracefully
    log.error("...");
}