Improve front-end HTTP error handling (#397)

* UI for HTTP error handing in diffexp route

* HTTP error handling for other routes

* clean up toasters

client/src/components/framework/toasters.js

@@ -1,12 +1,24 @@
 import { Position, Toaster, Intent } from "@blueprintjs/core";
 
 /** Singleton toaster instance. Create separate instances for different options. */
-export const ErrorToastTopCenter = Toaster.create({
+
+const ErrorToastTopCenter = Toaster.create({
   className: "recipe-toaster",
-  position: Position.TOP,
-  intent: Intent.WARNING
+  position: Position.TOP
 });
 
-export default {
-  ErrorToastTopCenter
-};
+/*
+A "user" error - eg, bad input
+*/
+export const postUserErrorToast = message =>
+  ErrorToastTopCenter.show({ message, intent: Intent.WARNING });
+
+/*
+a hard network error
+*/
+export const postNetworkErrorToast = message =>
+  ErrorToastTopCenter.show({
+    message,
+    timeout: 30000,
+    intent: Intent.DANGER
+  });