gen_component: add option to subscribe to exceptions
gen_component: add option to subscribe to exceptions

this is meant as a way to notify processes if a gen_component catches an
exception.
scalaris-team/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,jvf/scalaris,jvf/scalaris,jvf/scalaris,jvf/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,jvf/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,jvf/scalaris,scalaris-team/scalaris,jvf/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris