BUG/MEDIUM: cli: make sure we can report a warning from a bind keyword

BUG/MEDIUM: cli: make sure we can report a warning from a bind keyword

Since recent 2.5 commit c8cac04bd ("MEDIUM: listener: deprecate "process"
in favor of "thread" on bind lines"), the "process" bind keyword may
report a warning. However some parts like the "stats socket" parser
will call such bind keywords and do not expect to face warnings, so
this will instantly cause a fatal error to be reported. A concrete
effect is that "stats socket ... process 1" will hard-fail indicating
the keyword is deprecated and will be removed in 2.7.

We must relax this test, but the code isn't designed to report warnings,
it uses a single string and only supports reporting an error code (-1).

This patch makes a special case of the ERR_WARN code and uses ha_warning()
to report it, and keeps the rest of the existing error code for other
non-warning codes. Now "process" on the "stats socket" is properly
reported as a warning.

No backport is needed.