--audit crashes on non-existant files

[LouisTrezzini]

When you audit a baseline, if a file has been removed, it crashes the audit command and you lose all your progress.

Progress should be saved after each keystroke, and system calls should be protected by a try ... catch block

How to reproduce

1. Create a baseline with secrets
2. Remove one of the files referenced in the baseline
3. Audit the baseline

[KevinHock]

Here lies the relevant stack trace

Secrets Left: 1/1
Filename:     test_secrets/file_with_secrets.py
----------
sed: can't read test_secrets/file_with_secrets.py: No such file or directory
Traceback (most recent call last):
  File "/kevin/code/gh_detect_secrets/detect-secrets/three_six/bin/detect-secrets", line 11, in <module>
    sys.exit(main())
  File "/kevin/code/gh_detect_secrets/detect-secrets/three_six/lib/python3.6/site-packages/detect_secrets/main.py", line 43, in main
    audit.audit_baseline(args.audit[0])
  File "/kevin/code/gh_detect_secrets/detect-secrets/three_six/lib/python3.6/site-packages/detect_secrets/core/audit.py", line 40, in audit_baseline
    original_baseline['plugins_used'],
  File "/kevin/code/gh_detect_secrets/detect-secrets/three_six/lib/python3.6/site-packages/detect_secrets/core/audit.py", line 121, in _print_context
    plugin_settings,
  File "/kevin/code/gh_detect_secrets/detect-secrets/three_six/lib/python3.6/site-packages/detect_secrets/core/audit.py", line 212, in _get_secret_with_context
    filename,
  File "subprocess.py", line 336, in check_output
    **kwargs).stdout
  File "subprocess.py", line 418, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['sed', '-n', '1,9p', 'test_secrets/file_with_secrets.py']' returned non-zero exit status 2.
``

[KevinHock]

Fixed by #56