cloudtrail validate-logs will spend a long time doing nothing if log files do not exist for the date range

If I put the date for start time and end time in the future then `aws cloudtrail validate-logs` will spend a long time listing objects in the bucket.

## What I run:

`time aws cloudtrail validate-logs --region=eu-west-1 --trail-arn="arn:aws:cloudtrail:eu-west-1::trail/" --start-time=2525-01-01T0900 --end-time=2525-01-01T1000 --debug`

## What I expect:

```
Validating log files for trail arn:aws:cloudtrail:eu-west-1::trail/ between 2525-01-01T09:00:00Z and 2525-01-01T10:00:00Z

Results requested for 2525-01-01T09:00:00Z to 2525-01-01T10:00:00Z
No digests found
2.03 real 0.57 user 0.12 sys
```

If I try to validate logs for a time before CloudTrail being enabled, it takes about 2 seconds to recognise that there are no log files available.

## What I get:

```
Validating log files for trail arn:aws:cloudtrail:eu-west-1::trail/ between 2525-01-01T09:00:00Z and 2525-01-01T10:00:00Z

An error occurred (InvalidTimeRangeException) when calling the ListPublicKeys operation: Invalid input parameter! Start time is after current time
55.88 real 10.87 user 1.05 sys
```

Notice that this takes almost a minute real time, this is a lot longer than I would expect.

Running the command with --debug shows that the process spends a long time sending a lot of `ListObjects` requests.