…error

This was previously implemented at the GUI level but has been broken since #20640

Since #20640, the 'createTransaction' does no longer retrieve the
fee if the process fails due to insufficient funds.

But, since #25269, 'createTransaction' retrieves an error message
indicating that the total transaction amount exceeds the wallet
available balance when fees are included.

So this enum is no longer needed.

So it can be used by external modules without requiring
wallet.h dependency.

Bundle all function's outputs inside the util::Result returned object.

Reasons for the refactoring:
- The 'change_pos' ref argument has been a source of bugs in the past.
- The 'fee' ref argument is currently only set when the transaction creation process succeeds.