Better handle bucket name length limits

http://docs.aws.amazon.com/AmazonS3/latest/dev/BucketRestrictions.html#bucketnamingrules
```
Bucket names must be at least 3 and no more than 63 characters long
```

Right now we just throw a generic bucket name error:

```
botocore.exceptions.ClientError: An error occurred (InvalidBucketName) when calling the CreateBucket operation: The specified bucket is not valid.
```

We can probably do better and catch some of these rules earlier.