Extend failure reasons for "system failure"

Currently any write failures during updating the lease will be signaled as observer error.

Message: Request timed out.

ActivityId: 4ff3e373-d013-4b17-8a49-8bf809fe37e4, Request URI: /apps/5933d010-85f0-4cb0-8bf8-2178969cd8f4/services/6af65582-1b70-43c5-9b0f-2fb075657719/partitions/c8dde228-5c34-4342-b3a0-93d782199bca/replicas/131826275780005884p/, RequestStats: , SDK: documentdb-dotnet-sdk/2.0.0 Host/64-bit MicrosoftWindowsNT/6.2.9200.0

at Microsoft.Azure.Documents.Rntbd.Channel.d__15.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

at Microsoft.Azure.Documents.Rntbd.LoadBalancingChannel.d__19.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

at Microsoft.Azure.Documents.Rntbd.TransportClient.d__5.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

at Microsoft.Azure.Documents.ConsistencyWriter.d__13.MoveNext()

--- End of stack trace from previous location where exception was thrown ---

at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()