Properly check that result is an error

[khernyo]

Fixes a compile error in one of the tests on current rust nightly (1.36.0):

error: unused return value of `std::result::Result::<T, E>::is_err` that must be used
  --> src/crypto/key_pair.rs:48:3
   |
48 |   verify(&keypair.public, b"oops", Some(&sig)).is_err();
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |

[yoshuawuyts]

Ahh, yes. Excellent catch!