add check on get epoch from context
add check on get epoch from context

plutoshe/taskgraph,5kg/taskgraph,BaiGang/taskgraph,fengjingchao/taskgraph,taskgraph/taskgraph,5kg/taskgraph,kod3r/taskgraph,fengjingchao/taskgraph,kod3r/taskgraph,dzch/taskgraph,dzch/taskgraph,plutoshe/mapreduceOfTaskgraph,plutoshe/taskgraph,plutoshe/mapreduceOfTaskgraph,taskgraph/taskgraph,BaiGang/taskgraph