plugrack_use_by_type() threw away the plugin_err_t reported by

plugin_load_from_file(). Be sure to collect this error and
return a useful error message to the slurm log.

Additionally, even though I don't think plugin refcounts actually
do anything in SLURM, the refcount was inadvertently incremented
on PLUGIN_INVALID_HANDLE instead of for valid handles. This patch
fixes that bug as well.
(From Mark Grondona)