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));