perhaps when mypy should be re-run after --install-types, the exit code should be 3

[wyattscarpenter]

Frankly, this is a mess. Code 2 should not be used for --install-types or blocking errors IMO - 2 is de-facto standard for "please do not invoke me like this". "Run again" and "invalid syntax" have so little in common that I struggle to understand the reason to combine them under one code.

Originally posted by @sterliakov in #19548

It would probably be acceptable to pick whatever we want to be the exit code --install-types gives when mypy needs to be re-run afterwards, but 3 seems a good choice.

[wyattscarpenter]

Maybe 3 should be for mypy internal errors (what do these currently return?) and 4 should be for the install types thing 🤔