don't td.FailStep() quite yet

Calling td.FailStep() as written here doesn't really do anything except
hide the more useful summary error, e.g. "484 runs of build/fm failed
after retries."  Maybe it'll become useful again if I add step nesting?

Change-Id: I23eb59afce8559f4b0e549f31873577939fc7ca7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/364497
Reviewed-by: Mike Klein <mtklein@google.com>

infra/bots/task_drivers/fm_driver/fm_driver.go

@@ -170,8 +170,6 @@ func main() {
 					cmd.Name,
 					strings.Join(cmd.Args, " "),
 					strings.Join(lines, "\n\t"))
-			} else {
-				td.FailStep(ctx, err)
 			}
 			return
 		}