Display errors about messages not being sent near the associated message
Considering Alice sends message to Carol. From Gajim chat window:
... [21:23:17] alice: 7 [23:47:51] error while sending message ( recipient-unavailable ) [23:47:51] carol/77ab1225-7e9d-4187-9d11-24983ff5d9b3 is now Offline
The error looks useless: you don't know what of previously sent messages has not been delivered. Carol uses Conversations and receives "7" successfully (OK, this is another issue).
Gajim sent the message to local prosody 0.9.12 server (with MAM):
01/29/2017 21:23:17 (I) nbxmpp.client_nb raising event from transport: :::::DATA SENT:::: _____________ <message xmlns="jabber:client" to="carol@testhost/77ab1225-7e9d-4187-9d11-24983ff5d9b3" type="chat" id="79a79e09-6a42-42d2-a966-0e40e35d899d"><body>7</body><thread>osAeZoBvwJAGssqfPBIcGnnFsptTuNDB</thread></message> _____________
Gajim receives the message:
01/29/2017 23:47:51 (I) nbxmpp.client_nb raising event from transport: :::::DATA RECEIVED:::: _____________ <message id='79a79e09-6a42-42d2-a966-0e40e35d899d' type='error' to='alice@testhost/Gajim' from='carol@testhost/77ab1225-7e9d-4187-9d11-24983ff5d9b3'><error type='cancel'><recipient-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/></error></message><presence type='unavailable' to='alice@testhost' from='carol@testhost/77ab1225-7e9d-4187-9d11-24983ff5d9b3'/> _____________
So Gajim does know what message exactly could not be delivered thanks to "Message ID" (despite it has been, but this is perhaps issue with either prosody or jabber as protocol) and instead of citing time and body and telling this is likely to not be delivered, issues only useless error message.