Avoid server crash when handler returns LspResult.Error (#33)

It appears that #29 broke the reporting of
LspResult.Error to StreamJsonRpc.

StreamJsonRpc processes LocalRpcException to report exceptions/errors from request handlers.
In particular we have code in `requestHandling` that actually throws LocalRcpException when
handler returns LspResult.Error.

This commit adds code to skip markingLocalRcpException as a fatal exception and make it not
crash the server.

src/LanguageServerProtocol.fs

@@ -90,7 +90,11 @@ module Server =
     // and thus any exception that happens during e.g. text sync gets swallowed.
     use jsonRpc =
       { new JsonRpc(jsonRpcHandler) with
-          member this.IsFatalException(ex: Exception) = true }
+          member this.IsFatalException(ex: Exception) =
+              match ex with
+              | :? LocalRpcException -> false
+              | _ -> true
+      }
 
     /// When the server wants to send a notification to the client
     let sendServerNotification (rpcMethod: string) (notificationObj: obj) : AsyncLspResult<unit> =