Temporary fix for high CPU usage when stream lost

This change introduces a "fix" for the issue where the language service
host process will spin at high CPU if the client's input stream terminates
unexpectedly.  This issue is caused by incorrectly handling the result of
Stream.ReadAsync which states that a result of 0 means that the stream has
terminated.  Instead of returning true and continuing the read loop, I now
throw an EndOfStreamException which causes the app to terminate with an
unhandled exception.

A future fix will be implemented to provide a more graceful shutdown path.

Author: daviwil

src/PowerShellEditorServices.Protocol/MessageProtocol/MessageReader.cs

@@ -142,7 +142,18 @@ await this.inputStream.ReadAsync(
 
             this.bufferEndOffset += readLength;
 
-            return readLength >= 0;
+            if (readLength == 0)
+            {
+                // If ReadAsync returns 0 then it means that the stream was
+                // closed unexpectedly (usually due to the client application
+                // ending suddenly).  For now, just terminate the language
+                // server immediately.
+                // TODO: Provide a more graceful shutdown path
+                throw new EndOfStreamException(
+                    "MessageReader's input stream ended unexpectedly, terminating.");
+            }
+
+            return true;
         }
 
         private bool TryReadMessageHeaders()