Change LIBSODIUM_ERROR to -1.

We only relied on success being 0 and our code was otherwise agnostic to the
actual return code in the event of failed signature verification, but this
change keeps the API consistent.

src/gtest/test_consensus.cpp

@@ -48,7 +48,7 @@ void ZIP215Check(
     }
 
     if (!valid_zip215) {
-        expected_zip215 = 1;
+        expected_zip215 = -1;
     }
 
     EXPECT_EQ(

src/rust/src/rustzcash.rs

@@ -1345,7 +1345,7 @@ pub extern "system" fn librustzcash_mmr_hash_node(
 // mimic the `crypto_sign_verify_detached` API in libsodium, but uses
 // the ed25519-zebra crate internally instead.
 const LIBSODIUM_OK: isize = 0;
-const LIBSODIUM_ERROR: isize = 1;
+const LIBSODIUM_ERROR: isize = -1;
 
 #[no_mangle]
 pub extern "system" fn librustzcash_zebra_crypto_sign_verify_detached(