MINOR: cli: display failure reason on wait command

MINOR: cli: display failure reason on wait command

wait CLI command can be used to wait until either a defined timeout or a
specific condition is reached. So far, srv-removable is the only event
supported. This is tested via srv_check_for_deletion().

This is implemented via srv_check_for_deletion(), which is
able to report a message describing the reason if the condition is
unmet.

Previously, wait return a generic string, to specify if the condition is
met, the timer has expired or an immediate error is encountered. In case
of srv-removable, it did not report the real reason why a server could
not be removed.

This patch improves wait command with srv-removable. It now displays the
last message returned by srv_check_for_deletion(), either on immediate
error or on timeout. This is implemented by using dynamic string output
with cli_dynmsg/dynerr() functions.