exec_resource_agent: return valid exit code instead of die's

exec_resource_agent: return valid exit code instead of die's

Switch from die's to logging and return the respective exit codes.
This adds the possibility to handle (i.e.: fix) some errors outside
of the forked exec_resource_agent worker.

This does not changes behaviour for now, as the die returned an 255
exit code. We didn't checked on that exit code explicitly and so we
are safe to use the new exit codes, it results in the same behaviour
for the other code (most important the CRM Manager class).

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>