Improve error message for minimal headers validation

[by22Jy]

Summary

• Improves the error message when the number of headers provided is less than the minimal required
• Adds a helpful hint to use -m/--minimal-headers option

Context

Fixes #974

Users reported being confused when they got an error saying they need at least 4 headers, without knowing they can adjust this threshold with the --minimal-headers option.

Example:

$ mdedup -h X-Pm-Gluon-Id ...
Error: Invalid value: Provided number of headers to hash (1) is less than the minimal required number of headers (4).

With this change:

$ mdedup -h X-Pm-Gluon-Id ...
Error: Invalid value: Provided number of headers to hash (1) is less than the minimal required number of headers (4). Use -m/--minimal-headers to allow fewer headers.

Changes

- f"the minimal required number of headers ({minimal_headers})."
+ f"the minimal required number of headers ({minimal_headers}). "
+ f"Use -m/--minimal-headers to allow fewer headers."

Testing

The change only affects the error message text, no functional changes.