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).

plugins: imap-filter-sieve: cmd-filter-sieve - Avoid using imap_arg_as_nstring() in cmd_filter_sieve_script_parse_value_arg()

The argument can only be a quoted string here, so imap_arg_as_string() will suffice.

This was not supposed to be accepted and led to unexpected behavior:

- FILTER SIEVE PERSONAL NIL was handled as FILTER SIEVE DELIVERY,
- FILTER SIEVE GLOBAL NIL caused an internal error.