Validate chat ID before joining

Only add a new chat conversation after we ensured that the chat ID
exists.

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>
Display an error message when trying to join a chat

Pidgin currently crashes if the user tries to join a chat by using the
"Join a Chat..." feature of Pidgin. Since WhatsApp does not allow for
joining a chat by name at all, display an error message instead.

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>
Split out chat participant retrieval

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>
Merge pull request #42 from cryptocrack/chat-join-fixes

Chat join fixes