[codemod] add integration test for show errors from formatter subprocess call

[jimmylai]

Summary

Without the fix, it shows error like this and it's not useful to understand the cause.

Traceback (most recent call last):
  File "/Users/jimmylai/github/LibCST/libcst/codemod/_cli.py", line 334, in _parallel_exec_process_stub
    newcode = invoke_formatter(formatter_args, newcode)
  File "/Users/jimmylai/github/LibCST/libcst/codemod/_cli.py", line 63, in invoke_formatter
    encoding=None if work_with_bytes else "utf-8",
  File "/Users/jimmylai/anaconda3/lib/python3.7/subprocess.py", line 395, in check_output
    **kwargs).stdout
  File "/Users/jimmylai/anaconda3/lib/python3.7/subprocess.py", line 487, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['/Users/jimmylai/github/libcst-env/bin/black', '-']' returned non-zero exit status 123.

Failed to codemod /Users/jimmylai/github/LibCST/libcst/codemod/tests/codemod_formatter_error_input.py

With the fix, it includes those detail to make debug easy.

Codemodding /Users/jimmylai/github/LibCST/libcst/codemod/tests/codemod_formatter_error_input.py
# Copyright (c) Facebook, Inc. and its affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.
#
# pyre-strict

from contextlib import AsyncExitStack


def fun():
    # this is an explicit syntax error to cause formatter error
    async with AsyncExitStack() as stack:
        pass

error: cannot format -: Cannot parse: 13:10:     async with AsyncExitStack() as stack:

This is for fixing issue @lexprfuncall had.

Test Plan

added test case.