Token mint screen should check if already minted

[the-frey]

Related to #32

If you spam the token lookup screen, sometimes it's slow enough to return 'minted' that a user can land on the mint screen.

Of course there are also ways of getting there via routing too.

Mint screen should check if a token id is minted and error if so.

The contract will do the right thing, but it could result in users accidentally burning fees (although most will probably be malicious in this case).