Continue checking all monitors after sending alert

Previously this was mistakenly returning after sending an alert. Now
all alerts will be sent unless there is an exception on one of them.

main.go

@@ -79,7 +79,11 @@ func checkMonitors(config *Config) error {
 			Metrics.CountCheck(monitor.Name, success, monitor.LastCheckMilliseconds(), hasAlert)
 
 			if alertNotice != nil {
-				return sendAlerts(config, monitor, alertNotice)
+				err := sendAlerts(config, monitor, alertNotice)
+				// If there was an error in sending an alert, exit early and bubble it up
+				if err != nil {
+					return err
+				}
 			}
 		}
 	}