This commit fixes a bug when opal_error_init is called with the same
values multiple times. If opal_error_init is called too many times it
will start failing with OPAL_ERR_OUT_OF_RESOURCE. To fix the problem
check if an existing convertor matching the requested one and return
that one instead.
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>