Makes sure it is consistent for a later commit when action exection is more fragmented.

mailbox_get_last_error() should be used only for client-visible errors.
The internal errors should be used for logging.

This indicates whether the last script in the sequence is being executed.

Default is now debug logging, which is used for the IMAP plugins. The LDA Sieve
plugin uses info logging.

This way, the execution order can be made explicit.

Addresses a Coverity report.

Before it always used the name of the action definition, which is confusing e.g.
for the keep and fileinto actions, which both map to the same "store" action
definition.

This allows logging action messages through a different (child) event.