Consistently check Status.getStackDump() is non-empty in exception message construction

[joeshannon]

The Status (org.epics.pvdata.pv.Status) interface defines getStackDump which returns a string.

This is used in several places for logging purposes or to construct other exceptions.

When working with RPCClientImpl we are seeing that the RPCRequestException contains a superfluous ", cause:\n". This is because the string from getStackDump() is non-null but an empty string but only non-null is considered in RPCClientImpl. In other places the string is already checked to be empty. Here is a summary of the callers:

• org.epics.pvaccess.server.impl.remote.tcp.BlockingServerTCPTransport.authenticationCompleted(Status)
• org.epics.pvaccess.server.impl.remote.tcp.NonBlockingServerTCPTransport.authenticationCompleted(Status)
• org.epics.pvaccess.server.impl.remote.rpc.ServerRPCService.OpChannels.execute()
• org.epics.pvaccess.client.impl.remote.handlers.CreateChannelHandler.handleResponse(InetSocketAddress, Transport, byte, byte, int, ByteBuffer)
• org.epics.pvaccess.client.rpc.RPCClientImpl.request(PVStructure, double)
• org.epics.pvaccess.impl.remote.tcp.BlockingTCPTransport.verified(Status)
• org.epics.pvaccess.impl.remote.tcp.NonBlockingTCPTransport.verified(Status)

All apart from RPCClientImpl and ServerRPCService already check this.