CloudWatch: Improved error handling when throttling occurs

[sunker]

The Cloudwatch API might return a throttling error either when the transactions per second (TPS) or the Datapoints Per Second (DPS) is reached. See documentation In the case that happens, it should be clear to the user what has happened.

When throttling occurred, an alert should be displayed in the dashboard/query editor. There should be one alert per combination datasource name / region. The alert should inform the user that has happened and display a link to the cloudwatch documentation.