Modify text of error message for required field I have a checkbox with a link in its title text. This is a required field and the error message references the title including the link. How can I modify the error message to remove the link from the error message text? 

hook_field_validate() appears not to be called; that was my first attempt after doing some searching.
Quick and easy fix is to use the String Overrides module. Here is a quote about it (from the module's project page):

Provides a quick and easy way to replace any text on the site.
Features:

Easily replace anything that's passed through t()
Locale support, allowing you to override strings in any language
Ability to import/export *.po files, for easy migration from the Locale module
Note that this is not a replacement to Locale as having thousands of overrides can cause more pain then benefit. Use this only if you need a few easy text changes.