Use a more specific exception class for note decryption failure

Closes #1545.

src/zcash/NoteEncryption.cpp

@@ -129,7 +129,7 @@ typename NoteDecryption<MLEN>::Plaintext NoteDecryption<MLEN>::decrypt
                                              NULL,
                                              0,
                                              cipher_nonce, K) != 0) {
-        throw std::runtime_error("Could not decrypt message");
+        throw note_decryption_failed();
     }
 
     return plaintext;

src/zcash/NoteEncryption.hpp

@@ -82,6 +82,11 @@ public:
 uint256 random_uint256();
 uint252 random_uint252();
 
+class note_decryption_failed : public std::runtime_error {
+public:
+    note_decryption_failed() : std::runtime_error("Could not decrypt message") { }
+};
+
 }
 
 typedef libzcash::NoteEncryption<ZC_NOTEPLAINTEXT_SIZE> ZCNoteEncryption;