Handling errors in the Article Editor

[SudShekhar]

First of all, thanks for this making this. It clarified a lot of react/redux concepts for me.

I opened this PR to point out that

• /editor page can be accessed without logging in
• Clicking submit on this page without logging in causes the app to throw an error
• Even if you're logged in and you click submit without filling out all the form fields, the app throws an error

Main reasons, in reducers/common.js

case 'ARTICLE_SUBMITTED':
      const redirectUrl = `article/${action.payload.article.slug}`;
      return { ...state, redirectTo: redirectUrl };

When ARTICLE_SUBMITTED doesn't succeed, there is no article returned and an error is thrown. This might work better

case 'ARTICLE_SUBMITTED':
        if(!action.error){
          const redirectUrl = `article/${action.payload.article.slug}`;
          return { ...state, redirectTo: redirectUrl };
        }
        break;

In reducers/editor.js, when you're not logged in the server returns a 401 and an error occurs below (Since the payload is empty):

case 'ARTICLE_SUBMITTED':
      return {
        ...state,
        inProgress: null,
        errors: action.error ? action.payload.errors : null
      };

Changing the above to this works:

case 'ARTICLE_SUBMITTED':
         return {
            ...state,
            inProgress: null,
            errors: action.error? 
               (action.payload? action.payload.errors: ["Unauthorized"]) : null
         };

I've hardcoded "Unauthorized" here but I guess there might be much better approaches for this.
Just my two cents. Thanks for the amazing tutorial.