Do not throw if a plugin fails to load.
Do not throw if a plugin fails to load.

- If the call to geopm_plugin_register returns an error code
  the runtime used to completely fall over.
- Now the runtime will print a warning if the library is configured
  with --enable-debug.
- When not configured for debug, a failure in plugin registration
  is quietly ignored.

Change-Id: Ibf2af65f3de9672e9e89c9ea8fdce16e209b97f2
Signed-off-by: Christopher M. Cantalupo <9cf3ecca60a3dd7112ab0e7250bedb8a752e3958@intel.com>

geopm/geopm,geopm/geopm,cmcantalupo/geopm,cmcantalupo/geopm,geopm/geopm,cmcantalupo/geopm,cmcantalupo/geopm,cmcantalupo/geopm,geopm/geopm,geopm/geopm