Avoid crashing on invalid python executables

[pranavrajpal]

Description

When an invalid python executable is passed to mypy, show an error message instead of crashing with a FileNotFoundError or some other exception.

The implementation of the error raises a CompileError instead of using the normal error handling from mypy.errors.Errors, similar to what we do for nonexistent files being passed to mypy:

mypy/mypy/build.py

Lines 2058 to 2061 in 8e7e817

	raise CompileError([
	"mypy: can't read file '{}': {}".format(
	self.path, os.strerror(ioerr.errno))],
	module_with_blocker=self.id) from ioerr

I used that approach because the relevant code didn't have access to an Errors instance and there probably isn't much reason to support continuing to check code when the python_executable is incorrect.

Test Plan

I couldn't reproduce the crash in the test suite (I'm guessing that's because we don't try looking for site-packages in the test suite, but I haven't confirmed that), so I tested this manually by running mypy empty.py --python-executable nonexistent, where empty.py is an empty file.

On master (8e7e817):

Traceback (most recent call last):
  File "/home/pranav/.local/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/pranav/.local/lib/python3.8/site-packages/mypy/__main__.py", line 12, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "/home/pranav/.local/lib/python3.8/site-packages/mypy/main.py", line 96, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/home/pranav/.local/lib/python3.8/site-packages/mypy/main.py", line 173, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/pranav/.local/lib/python3.8/site-packages/mypy/build.py", line 181, in build
    result = _build(
  File "/home/pranav/.local/lib/python3.8/site-packages/mypy/build.py", line 213, in _build
    search_paths = compute_search_paths(sources, options, data_dir, alt_lib_path)
  File "/home/pranav/.local/lib/python3.8/site-packages/mypy/modulefinder.py", line 788, in compute_search_paths
    egg_dirs, site_packages = get_site_packages_dirs(options.python_executable)
  File "/home/pranav/.local/lib/python3.8/site-packages/mypy/modulefinder.py", line 653, in get_site_packages_dirs
    subprocess.check_output([python_executable, pyinfo.__file__, 'getsitepackages'],
  File "/usr/lib/python3.8/subprocess.py", line 415, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.8/subprocess.py", line 493, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.8/subprocess.py", line 858, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.8/subprocess.py", line 1704, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'nonexistent'

With this PR:

mypy: Invalid python executable 'nonexistent': No such file or directory

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JukkaL]

Thanks! This is much more friendlier than showing a traceback.