Handle invalid contractId on /login/ request

[kcole16]

Problem
If the contractId is wrong in a /login/ (e.g. login/?contractId=THISDOESNTEXIST) request from an app, a useless access key is created (or is it?) and the app is forced to identify the error with little to no information.

Expected Behavior
If a contractId is invalid, we should prevent the user from going through the /login/ step, and return an error to the app.

Steps to reproduce

1. Go to corgis.nearprotocol.com
2. Select "Login w/ Near"
3. Edit the contractId argument in the url to contractId=notrealid and resubmit
4. Complete login
5. Try to generate a Corgi

[kcole16]

Acceptance Criteria

Login /login

• If contractId does not exist, the user is displayed a warning message (see below) in a modal
• Clicking "Return to App" in the modal redirects the user to the referring application

Warning Message

"The contract <contractId> does not exist. This may be an issue with the application you are trying to use, or this contract may have been deleted. You will be redirected back to the application"

[vgrichina]

question from @marcinbodnar

how can we check if contract_id from param is valid?

It has same constraints as account Id + it also needs to be an existing account (i.e. we'd want to query near node for that account). Can be done similarly to user profile component I think (after Chad changed it to work with any account vs current account).