BUG/MINOR: ring: size warning incorrectly reported as fatal error

BUG/MINOR: ring: size warning incorrectly reported as fatal error

When a ring section defines its size using the "size" directive with a
smaller size than the default one or smaller than the previous one,
a warning is generated to inform the user that the new size will be
ignored.

However the err_code is returned as FATAL, so this cause haproxy to
incorrectly abort the init sequence.

Changing the err_code to ERR_WARN so that this warning doesn't refrain
from successfully starting the process.

This should be backported as far as 2.4