Return result of safely_notify/1 from safely_histogram_timed_notify/1

This allows us to lazy create metrics based on {error, Name,
nonexistent_metric}

1> application:ensure_all_started (folsom).
{ok,[bear,folsom]}
2> Timer = folsom_metrics:histogram_timed_begin(foo_hist).
{foo_hist,{1416,136479,976526}}
3> folsom_metrics:safely_histogram_timed_notify(Timer).
{error,foo_hist,nonexistent_metric}
4>