Update lefthook error message for gettext

What does this MR do and why?

Add clarifying output suggesting how dev can solve gettext error when running lefthook run pre-push with outdated PO files.

Related to Clarify documentation on updating PO files (!137179 - merged)

How to set up and validate locally

1. Check out this branch locally
2. Create a new branch from this branch (name it whatever you like, it will be discarded after)
3. Make an edit to any externalized string in a vue file -- any string that is wrapped in s__(...) will do.
4. Commit your change (a change like this needs to be in an un-pushed local commit to test the functionality changed in this MR)
5. run lefthook run pre-push on the command line
   • You should get an error, because lefthook's gettext check will fail. Beneath the failing gettext check output, you should see a line that says PO files need to be updated. Try running bin/rake gettext:regenerate
6. Check this branch out again and delete your branch from step 2

Here's an example of the sort of output you should see:

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.