IqError is now caught and forwarded to the command error handler referenced
in the session.
Errors are now caught and processed by the session's error handler
whether or not the results Iq stanza includes the <command> substanza.
Added the option for blocking command calls. The blocking option is set
during start_command with block=True. Subsequent command flow methods use
session['block'] to determine their blocking behaviour.
If you use blocking commands, then you will need to wrap your command calls
in a try/except block for IqTimeout exceptions.
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