schnorr: Change verify_schnorr error return type

From InvalidSignature to IncorrectSignature; the former prints the
message "malformed signature", which doesn't represent the correct
error.

src/schnorr.rs

@@ -181,7 +181,7 @@ impl<C: Verification> Secp256k1<C> {
             if ret == 1 {
                 Ok(())
             } else {
-                Err(Error::InvalidSignature)
+                Err(Error::IncorrectSignature)
             }
         }
     }