Use fmt to convert whatever's in error to a string.

This way we don't crash if someone sticks something else in the error
key in the state bag (which a quick glance at the code tells me we're
already doing.

Perhaps in the future we can add an error attribute to the state bag
but for now this will have to suffice.

common/multistep_runner.go

@@ -73,7 +73,7 @@ func (s abortStep) Run(ctx context.Context, state multistep.StateBag) multistep.
 func (s abortStep) Cleanup(state multistep.StateBag) {
 	err, ok := state.GetOk("error")
 	if ok {
-		s.ui.Error(err.(error).Error())
+		s.ui.Error(fmt.Sprintf("%s", err))
 	}
 	if _, ok := state.GetOk(multistep.StateCancelled); ok {
 		s.ui.Error("Interrupted, aborting...")
@@ -105,7 +105,7 @@ func (s askStep) Run(ctx context.Context, state multistep.StateBag) (action mult
 
 		err, ok := state.GetOk("error")
 		if ok {
-			s.ui.Error(err.(error).Error())
+			s.ui.Error(fmt.Sprintf("%s", err))
 		}
 
 		switch ask(s.ui, typeName(s.step), state) {