Description

Add validation to detect when referenced commits are not found in the
repository, with special handling for Linux kernel tags.

Changes:

• Add LinuxKernelTagNotPushedError exception for distinguishable error handling
• Check if old_commit exists in repository before fetching commit range
• Detect Linux kernel tag format (vX.Y.Z) and provide helpful hint message
• Continue processing all branches instead of stopping at first error
• Collect all missing commit errors and report them together at the end
• Format error output with prominent "ERROR:" prefix and branch name
• Exit with error code 1 when any commits are not found

This prevents the script from proceeding with invalid data and ensures
all unpushed tags are reported in a single run, making it easier for
users to identify and fix all issues at once.

Example output:
ERROR: eve-kernel-amd64-v6.12.49-generic: commit 'v6.12.49' is not
found in the repository. This appears to be a Linux kernel tag
(vX.Y.Z format) that must be pushed to the repository first.

How to test and validate this PR

not easy, you need to have a newly pushed branch. @rene has one right now and can validate it

Changelog notes

None

PR Backports

Not requires because we do not introduce new kernels on old branches

Checklist

• I've provided a proper description
• I've added the proper documentation
• I've tested my PR on amd64 device
• I've tested my PR on arm64 device
• I've written the test verification instructions
• I've set the proper labels to this PR

And the last but not least:

• I've checked the boxes above, or I've provided a good reason why I didn't
  check them.