refactor: improve key transport message handling and deduplicate errors

Centralize heartbeat detection and have a function for error to string
mapping. So we don't need the same code in multiple handlers.

Signed-off-by: Michael Vetter <jubalh@iodoru.org>

src/omemo/omemo.c

@@ -1147,6 +1147,7 @@ omemo_on_message_recv(const char* const from_jid, uint32_t sid,
 
     if (payload == NULL) {
         signal_buffer_free(plaintext_key);
+        *error = OMEMO_ERR_KEY_TRANSPORT;
         return NULL;
     }