mypy --install-types Usability Issue

Summary

When no .mypy_cache folder exists and a user runs mypy --install-types --non-interactive ./, mypy may fail on fundamental Python issues but suggest that the failure was because of "no mypy cache directory," confusing the user.

Reproduction

Environment

Ubuntu 20, Python 3.6, mypy 0.910. (The old Python version reflects the setup of a production codebase where this issue was discovered.)

Confirm The Failure

Using this repo as-is, first run main.py to confirm it works.

$ python3 main.py
test

Then, run mypy.

$ mypy --version
mypy 0.910
$ mypy --install-types --non-interactive ./
error: --install-types failed (no mypy cache directory)

We can provoke success by creating an empty cache directory and running mypy again.

$ mkdir .mypy_cache
$ mypy --install-types --non-interactive ./
b/lib.py: error: Duplicate module named "lib" (also at "./a/lib.py")
b/lib.py: note: Are you missing an __init__.py? Alternatively, consider using --exclude to avoid checking one of them.
Found 1 error in 1 file (errors prevented further checking)

Address the errors, remove .mypy_cache, and run mypy again.

$ touch a/__init__.py
$ touch b/__init__.py
$ rm -r .mypy_cache/
$ mypy --install-types --non-interactive ./
main.py:2: error: Incompatible types in assignment (expression has type "str", variable has type "int")
Found 1 error in 1 file (checked 5 source files)