[To-Do] Add FAQ crosslink for Input Error Messages

This issue is a part of the PR: #3404

_Need to crosslink all the instances of Understanding input error messages with other sub-FAQs in GTN._
Mentioning @jennaj to have a look at this.

Review PR #3404 first, since this issue says it is part of that work. Locate every instance of “Understanding input error messages” in the GTN training material and compare its surrounding FAQ links. Done means each instance is crosslinked with the relevant sub-FAQs.

Written by the indexing model from the issue text.