[Server][6.3.2] Improve ALREADY_EXISTS status handling

[jerrymarino]

Sending an ALREADY_EXISTS for duplicated writes. Sometimes we may have this value after the client makes a find missing blobs request. In 6.4.0rc1 there were a few improvements to this by the way. It seems like this is a better treatment of it regardless

This also fixes an issue with some serious log spam.

[jerrymarino]

I was having a trace like this:

build.buildfarm.common.Write$WriteCompleteException
at build.buildfarm.common.grpc.StubWriteOutputStream.write(StubWriteOutputStream.java:248)
at java.base/java.io.OutputStream.write(OutputStream.java:124)
at com.google.protobuf.ByteString$LiteralByteString.writeTo(ByteString.java:1381)
at build.buildfarm.common.services.WriteStreamObserver.writeData(WriteStreamObserver.java:420)
at build.buildfarm.common.services.WriteStreamObserver.handleWrite(WriteStreamObserver.java:392)
at build.buildfarm.common.services.WriteStreamObserver.handleRequest(WriteStreamObserver.java:307)
at build.buildfarm.common.services.WriteStreamObserver.initialize(WriteStreamObserver.java:250)
at build.buildfarm.common.services.WriteStreamObserver.onUncommittedNext(WriteStreamObserver.java:128)
at build.buildfarm.common.services.WriteStreamObserver.onNext(WriteStreamObserver.java:107)
at build.buildfarm.common.services.WriteStreamObserver.onNext(WriteStreamObserver.java:56)
at io.grpc.stub.ServerCalls$StreamingServerCallHandler$StreamingServerCallListener.onMessage(ServerCalls.java:262)
at io.grpc.ForwardingServerCallListener.onMessage(ForwardingServerCallListener.java:33)
at io.grpc.util.TransmitStatusRuntimeExceptionInterceptor$1.onMessage(TransmitStatusRuntimeExceptionInterceptor.java:65)
at io.grpc.ForwardingServerCallListener.onMessage(ForwardingServerCallListener.java:33)
at io.grpc.Contexts$ContextualizedServerCallListener.onMessage(Contexts.java:76)
at io.grpc.internal.ServerCallImpl$ServerStreamListenerImpl.messagesAvailableInternal(ServerCallImpl.java:333)
at io.grpc.internal.ServerCallImpl$ServerStreamListenerImpl.messagesAvailable(ServerCallImpl.java:316)
at io.grpc.internal.ServerImpl$JumpToApplicationThreadServerStreamListener$1MessagesAvailable.runInContext(ServerImpl.java:835)
at io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
at io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:133)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
at java.base/java.lang.Thread.run(Thread.java:1589)

[werkt]

Conditional this fine logging with if (errorResponse... to be complete - errorResponse returns false if we've already delivered a response to the stream.

[jerrymarino]

SGTM - I've wrapped it in this conditional 👍