Remove trait UnwrapOrSgxErrorUnexpected

[clangenb]

This is a trait I introduced a long time ago without knowledge of how to handle errors in rust correctly. We should remove this unidiomatic trait bound and use the enclave error instead because:

• When logging the error with sgx_error_with_log, the error source is displayed as being in utils wich is not true. So important debugging information is hidden.
• it is unidiomatic.
• it is verbose, i.e., we need to call sgx_error()? compared to ?.