This is needed when the mailbox is opened with EXAMINE and is a source
for the operation. imap_sieve_mailbox_run_copy_source() expects the
source mailbox's context to exist.

Needed for adding a per-mailbox event in later commit.

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

It indicates that script execution exceeded a resource limit (currently only CPU
time limit).