638/error on login

[JaeGif]

This PR:

Resolves #638

• HTTP errors now handled using an extensible helper function.
• Errors raise a notification and reset the signup form.

[JaeGif]

This looks good! Was able to see the error response coming out from trying to register with the same email twice.

Was curious if we really want to clear the fields for email if the user failed to register though. Perhaps it could just be a silly mistake as to forgetting to type an extra character?

For example, someone could register with test11@test.com, but mistakenly typed test1@test.com instead, which someone else already registered with. Along with the error message, was thinking it might be easier for users to update the incorrect portions. A red outline around text field could help them focus on what field they need to correct, like:

But instead of Incorrect entry, it'll be test1@test.com already exist or something similar.

Let me know if you would like to update it here or do it with a follow-up PR. If you want to do it in a follow-up, I'll approve this for now.

Made it so the form remounts with old data if an error has occurred. Haven't added the color or bonus message yet.