Don't crash on un-stringable exceptions

[asottile]

Resolves #1358

I was able to reproduce and then fix the crash -- also fixed arbitrary errors in __str__ though I think those are nearly impossible

(the output below was retrieved by intentionally breaking virtualenv.py to print some garbage and then crash)

$ pre-commit run flake8 --all-files
[INFO] Initializing environment for https://gitlab.com/pycqa/flake8:astpretty==1,flake8-typing-imports==1.6.0.
[INFO] Installing environment for https://gitlab.com/pycqa/flake8.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
An unexpected error has occurred: CalledProcessError: command: ('/home/asottile/workspace/pre-commit/venv/bin/python3', '-mvirtualenv', '/home/asottile/.cache/pre-commit/repo0gp5o4dq/py_env-python3', '-p', '/home/asottile/workspace/pre-commit/venv/bin/python3')
return code: 1
expected return code: 0
stdout:
    ��
stderr: (none)
Check the log at /home/asottile/.cache/pre-commit/pre-commit.log