Improve submit_multiple_and_wait_verification return value

[avilagaston9]

In #1325, the output of submit_multiple_and_wait_verification was changed to a Vec<Result<_>> with:

• Ok() meaning that the proof was included in a batch.
• Err() meaning that the proof was not included due to an error.

But we are missing information about the verification result. If a timeout occurs while awaiting verification, an additional Err() is appended to the end of the returned array, which is somewhat unusual and does not provide information about the proof that triggers the error.