Linux 'troubleshoot' doesn't work on master

[joerick]

Just logging this so I don't forget.

I noticed in #322 that since #319 there's a error in the catch handler.

Traceback (most recent call last):
  File "/home/circleci/project/cibuildwheel/linux.py", line 260, in build
    dependency_install_flags='-c /constraints.txt' if options.dependency_constraints else '',
  File "/home/circleci/project/cibuildwheel/linux.py", line 20, in call
    args, input=input, universal_newlines=universal_newlines, check=True
  File "/usr/local/lib/python3.6/subprocess.py", line 438, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['docker', 'exec', '-i', 'cibuildwheel-7e9f032e-934e-4ec7-83c6-6731de41648f', '/bin/bash']' returned non-zero exit status 2.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/circleci/project/cibuildwheel/__main__.py", line 314, in <module>
    main()
  File "/home/circleci/project/cibuildwheel/__main__.py", line 224, in main
    cibuildwheel.linux.build(build_options)
  File "/home/circleci/project/cibuildwheel/linux.py", line 269, in build
    troubleshoot(options.project_dir, error)
AttributeError: 'BuildOptions' object has no attribute 'project_dir'