aws/request: Add new throttle error code (#2295)
aws/request: Add new throttle error code (#2295)

Adds a new throttle error code the SDK should react to when determing if
a request should be throttled.
aws/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go,jasdel/aws-sdk-go,yxd-hde/aws-sdk-go