Changelog:
===========
- Fixed error messages from :py:class:~aiohttp.resolver.AsyncResolver being
swallowed
- Added :exc:aiohttp.ClientConnectorDNSError for differentiating DNS resolution
errors from other connector errors
- Simplified DNS resolution throttling code to reduce chance of race conditions
Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>
