Fix the exception message for SetSeedFromMnemonic failure

It's possible for SetSeedFromMnemonic (a wrapper around zip339_phrase_to_seed) to fail for multiple reasons, including an
invalid language code or seed string -- a message about non-UTF8ness
is not correct.

src/zcash/address/mnemonic.h

@@ -79,7 +79,7 @@ public:
             READWRITE(mnemonic);
             language = (Language) language0;
             if (!SetSeedFromMnemonic()) {
-                throw std::ios_base::failure("Could not interpret the mnemonic phrase as a valid UTF-8 string.");
+                throw std::ios_base::failure("Invalid mnemonic phrase or language code.");
             }
         } else {
             uint32_t language0 = (uint32_t) language;