Include filename in error displayed if plugin file in baseline not found

[msabramo]

Fixes: #718

• Please check if the PR fulfills these requirements

• [x] Tests for the changes have been added (see test_plugin_not_found_in_baseline in tests/core/baseline_test.py)

• [ ] Docs have been added / updated

• [ ] All CI checks are green

• What kind of change does this PR introduce? Bug fix (for #718)

• What is the current behavior?

  718

• What is the new behavior (if this is a feature change)?

  $ detect-secrets-hook --baseline .secrets.baseline setup.py
  [initialize]    ERROR   Error: Failed to load `FakeCustomPlugin` plugin:
  [Errno 2] No such file or directory: './path/to/plugin/that/does/not/exist/plugin.py'
  [initialize]    ERROR   This error can occur when using a baseline that references a custom plugin
  with a path that does not exist.
  [scan]  ERROR   Unable to load plugins!

• Does this PR introduce a breaking change? No

• Other information:

[lorenzodb1]

@msabramo thank you for your contribution 😄 I'm going to need you to merge master into your branch to run the latest version of our CI

[msabramo]

@msabramo thank you for your contribution 😄 I'm going to need you to merge master into your branch to run the latest version of our CI

Sorry for the delay. Done in 3953579.