Error handling for EditContactUs page

[kwajiehao]

Currently, error handling for the EditContactUs page is lacking in sophistication.

1. We have multiple async API calls in the same try-catch block, so when one of them fails, we don't actually know which one is the cause.

      const settingsResp = await axios.get(`${process.env.REACT_APP_BACKEND_URL}/sites/${siteName}/settings`)
      const { footerContent, footerSha } = settingsResp.data.settings;
      
      const contactUsResp = await axios.get(`${process.env.REACT_APP_BACKEND_URL}/sites/${siteName}/pages/contact-us.md`);
      const { content, sha } = contactUsResp.data;

2. In the catch block, we simply redirect to the Not Found page if there are any errors (PR Feat/add 404 and generic error pages #280). We should redirect to the Not Found page only when the Contact Us file cannot be found (404 from the Contact Us page api call only). Any other error that arises should be dealt with with an error toast notification (PR Error-handling: toast notifications #279)