Change error enum for KeyPair::from_seckey_slice

Also does another nit in the file which removes a trailing whitespace

src/schnorrsig.rs

@@ -167,7 +167,7 @@ impl KeyPair {
         data: &[u8],
     ) -> Result<KeyPair, Error> {
         if data.is_empty() || data.len() != constants::SECRET_KEY_SIZE {
-            return Err(InvalidPublicKey);
+            return Err(InvalidSecretKey);
         }
 
         unsafe {