CLI 'validate' command should use exit codes and stderr properly

[donmccurdy]

Changes proposed:

• Return a non-zero exit code if validate command reports errors
• Errors and warnings that are not part of the validation report (CSV, markdown, etc.) should be written to stderr

Related:

• #1251
• #1420

These features would require changes in the CLI framework used by the library, not a quick fix at the moment.