feat: Provide descriptive fallback messages for OMEMO decryption failures

If an incoming OMEMO message failed to decrypt (due to missing session
keys or untrusted identities), Profanity would fall back to displaying
the raw XMPP body. This usually contained a generic string like "This
message is encrypted with OMEMO,".

We wrote the detailed reason in the debug logs but the user only saw the
fallback message and probably wondered *why* the message wasn't
displayed properly.

I'm unsure if we should display the fallback message as well though.

tests/unittests/omemo/stub_omemo.c

@@ -129,6 +129,27 @@ omemo_encrypt_file(FILE* in, FILE* out, off_t file_size, int* gcry_res)
 };
 void omemo_free(void* a) {};
 
+char*
+omemo_on_message_recv(const char* const from, uint32_t sid,
+                      const unsigned char* const iv, size_t iv_len,
+                      GList* keys, const unsigned char* const payload,
+                      size_t payload_len, gboolean muc, gboolean* trusted, omemo_error_t* error)
+{
+    if (error) {
+        *error = OMEMO_ERR_NONE;
+    }
+    return NULL;
+}
+
+char*
+omemo_receive_message(xmpp_stanza_t* const stanza, gboolean* trusted, omemo_error_t* error)
+{
+    if (error) {
+        *error = OMEMO_ERR_NONE;
+    }
+    return NULL;
+}
+
 uint32_t
 omemo_device_id()
 {