Give helpful error message if a string is used to name a GenServer / Supervisor

[alecspopa]

Hi, this is my second PR related to #3932.
It should add helpful messages for GenServer and Supervisor.
Thanks!

[michalmuskala]

I started thinking. Given in how many places we map the elixir-style process name to the erlang-style name (in standard library, and even more in 3rd party libraries), should we provide one function in the standard library to do this?

[josevalim]

@michalmuskala we already do as GenServer.whereis/1. This is slightly different because we need to pass the shape "unchanged" so it is invoked later on inside the process.

[whatyouhide]

Looks perfect to me! Thanks a lot @alecspopa 💟 :)

[alecspopa]

Thanks for merging @whatyouhide :)