Add blackd to nicely exit if missing aiohttp deps (#1761)

- If no aiohttp* deps exist nicely print a helpful message and exit
- There seems to be no nice way to optionally install the entry point, so lets make the entry point nicer

Test:
```
cooper-mbp1:black cooper$ /tmp/tb/bin/pip install .
cooper-mbp1:black cooper$ /tmp/tb/bin/blackd
aiohttp dependency is not installed: No module named 'aiohttp'. Please re-install black with the '[d]' extra install  to obtain aiohttp_cors: `pip install black[d]`
cooper-mbp1:black cooper$ /tmp/tb/bin/pip install .[d]
...
Successfully installed aiohttp-3.6.3 aiohttp-cors-0.7.0 black
cooper-mbp1:black cooper$ /tmp/tb/bin/blackd
blackd version 20.8b2.dev31+gdd2f86a.d20201013 listening on localhost port 45484
```

Fixes #1688