Adding Handling Tools for MultiException

[wiilei]

Two handling tools for MultiException have been added to reduce the complexity of dealing with nested exceptions when constructing complex ParSeq flows.

• allMultiCauses: Used to extract custom exceptions wrapped by MultiException, for example:

// Assuming both RPC and HTTP calls failed
Task<?> rpcRespTask = Task.par<RpcA, RpcB>.map(...);
Task<?> httpRespTask = Task.par<HttpA, HttpB>.map(...);
Task<?> task = Task.par(rpcRespTask, httpRespTask)
        .onFailure(e ->  // e: MultiException[MultiException[RpcAE, RpcBE], MultiException[HttpAE, HttpBE]]
                 Collection<Throwable> es = Exceptions.allMultiCauses(e);  // es: [RpcAE, RpcBE, HttpAE, HttpBE]

• anyMultiCause: Used to retrieve any single actual exception from MultiException, for example:

xxx.onFailure(e ->  // e: MultiException[MultiException[RpcAE, RpcBE], MultiException[HttpAE, HttpBE]]
                           // print: 'com.linkedin.parseq.MultiException (multiple causes follow; only first is shown in stack trace): ...'
                           Logger.error("has remote method error", e);

                           // print: 'com.xxx.RpcException: Rpc A Error ...'
                           Logger.error("has remote method error", Exceptions.anyMultiCause(e));