Improve error message on CRLF issues in committed migrations

[benjie]

Description

Fixes #224

The git conversion of LF to CRLF will cause issues not just with header parsing but also with hash signatures; as such we must require that these files are uncompromized. The error message now provides hints how to configure git to not mess with line endings for these files.

Performance impact

Negligible.

Security impact

Negligible.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.