Adds status parameter to sieve_result_execute().

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.