Add error handling in loading config file.
Add error handling in loading config file.

- Add error messages.
- Ignore fopen() failure in loading default config file as before.
- Terminate groonga if the format of config file is invalid.
- Terminate groonga if memory allocation fails in value registration.

hiroyuki-sato/groonga,naoa/groonga,redfigure/groonga,komainu8/groonga,cosmo0920/groonga,redfigure/groonga,myokoym/groonga,kenhys/groonga,naoa/groonga,hiroyuki-sato/groonga,myokoym/groonga,naoa/groonga,cosmo0920/groonga,groonga/groonga,komainu8/groonga,myokoym/groonga,redfigure/groonga,myokoym/groonga,kenhys/groonga,naoa/groonga,myokoym/groonga,cosmo0920/groonga,naoa/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,myokoym/groonga,hiroyuki-sato/groonga,kenhys/groonga,groonga/groonga,hiroyuki-sato/groonga,redfigure/groonga,naoa/groonga,cosmo0920/groonga,cosmo0920/groonga,cosmo0920/groonga,groonga/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,redfigure/groonga,naoa/groonga,redfigure/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,redfigure/groonga,myokoym/groonga,redfigure/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,hiroyuki-sato/groonga,komainu8/groonga,komainu8/groonga,myokoym/groonga,cosmo0920/groonga,komainu8/groonga,kenhys/groonga,hiroyuki-sato/groonga