Waiting proses update latest version WatchOS 4.3 the suddenly it appears an exclamation symbol,please apple care alert this issue!

Please see https://support.apple.com/en-us/HT204776

Also be aware Apple has just released Watch OS 4.3.1 in the last day or two, so it is possible you hit the moment when they were switching the new version over at the server. It might be worth trying again before you panic.