ERC-721 example: fix panic re: approve_for with nonexistent token

[zgrannan]

Summary

• [n] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependant on the specific version of cargo-contract or pallet-contracts?

In the current ERC-721 example used in integration tests (which is the source of the erc-721 example in ink-examples), the approve_for function will panic with the text '"Error with AccountId"' if the token id does not exist. This PR changes the function to return an Error::TokenNotFound in this case instead.

I am using Prusti to verify correctness properties of the contract; the presence of this bug prevents the verification of a "panic-freedom" property.

Description

Updated the logic and added a corresponding test.

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules No dependent changes

[ascjones]

LGTM

[ascjones]

Need to run cargo fmt to fix CI

Other instances of the bug

[ascjones]

There are a couple of other instances of calling self.owner_of which panic when None, could you fix those here too?

[zgrannan]

@ascjones done