Improve clarity of error messages in functions checking token ownership

🧐 Motivation Good morning folks. I feel this somewhat walks the line between feature and bug.

While extending some .*transfer() functionality in the OpenZeppelin contracts (thank you for providing them!) I found some of my tests failing due to an error reading:

ERC721: transfer caller is not owner nor approved

As a newcomer, most of the terminology of ownership in documentation and in the “higher level” contracts refers to the contract owner - with frequently used modifiers like onlyOwner and the Ownable contract reinforcing this. I spent quite some verifying that the test was indeed making calls with the owner’s signature, and that signature matches the .owner() in the instance of the contract under test.

After delving in a little more I found that the term “owner”, in this context, refers to the token owner.

(A more dramatic reading of this experience as at the bottom for those interested in such things)

📝 Details A small amount of clarification may go a long way to saving others the journey I went on myself, for example extending the error message to

ERC721: transfer caller is not *token* owner nor approved

I have a PR ready with this, and for all similar instances of such messages. Thank you for your consideration.

Dramatic Reading

“But I am the owner”, I whinge at the test suite, exhausted. In red, emblazoned in the flickering terminal: “‘ERC721: transfer caller is not owner nor approved’” stands its ground - repeatedly unconvinced of my ownership. I checksum, and I verify. The messages are signed with poise, grace, and cryptographic correctness. The deployer is requesting the transfer. The very deployer that is also the current owner of this fine establishment. So I dig. I fear I’ll dig too greedily and too deep, and awaken something in the darkness. “Ah.” I sigh. Owner can mean owner of the contract, but this “owner” refers to a check of owner of the token. So let this fable guide you all - don’t reuse curt terminology for something that is already overloaded. All this could have been avoided with a simple “ERC721: transfer caller is not token owner nor approved”.

[kanedaaaa]

Haha, yes I can send one.

[frangio]

@kanedaaaa I like that! Do you want to submit a PR?