Move error handling for taskSubmitter

taskSubmitter ended up in a confused state if the initial POST request
encountered an error.
I've updated `startPolling` to return a Promise, and moved the
error handling code to the top level.

This doesn't change anything for the case where we get a non-error
response to the POST request, but means that e.g. 400 errors
get caught and displayed properly.

openflexure_microscope/api/static/src/components/genericComponents/taskSubmitter.vue

@@ -175,19 +175,8 @@ export default {
         // Get the returned Task ID
         .then(response => {
           // Start the store polling TaskId for success
-          this.startPolling(response.data.id, response.data.href);
+          return this.startPolling(response.data.id, response.data.href);
-        });
+        })
-    },
-
-    startPolling: function(taskId, taskUrl) {
-      if (this.taskRunning != true) {
-        // Starts polling an existing Action task
-        this.taskId = taskId;
-        this.taskUrl = taskUrl;
-        // Start the store polling TaskId for success
-        this.taskRunning = true;
-        this.$emit("taskRunning", this.taskId);
-        this.pollTask(this.taskId, this.pollInterval)
         .then(response => {
           // Do something with the final response
 
@@ -211,6 +200,17 @@ export default {
             this.getFormData();
           }
         });
+    },
+
+    startPolling: function(taskId, taskUrl) {
+      if (this.taskRunning != true) {
+        // Starts polling an existing Action task
+        this.taskId = taskId;
+        this.taskUrl = taskUrl;
+        // Start the store polling TaskId for success
+        this.taskRunning = true;
+        this.$emit("taskRunning", this.taskId);
+        return this.pollTask(this.taskId, this.pollInterval);
       }
     },