fix golint issues, exported doc (#451)

core/errorx/atomicerror.go

@@ -2,14 +2,17 @@ package errorx
 
 import "sync/atomic"
 
+// AtomicError defines an atomic error.
 type AtomicError struct {
 	err atomic.Value // error
 }
 
+// Set sets the error.
 func (ae *AtomicError) Set(err error) {
 	ae.err.Store(err)
 }
 
+// Load returns the error.
 func (ae *AtomicError) Load() error {
 	if v := ae.err.Load(); v != nil {
 		return v.(error)