mrb logger: use raw backtrace entry as error message only when it's needed
mrb logger: use raw backtrace entry as error message only when it's needed

komainu8/groonga,komainu8/groonga,naoa/groonga,kenhys/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,groonga/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,naoa/groonga,naoa/groonga,naoa/groonga,kenhys/groonga,komainu8/groonga,naoa/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga