WIP: Extended error messages to detect missing formats and layouts. Related to #1746

Changed error messages if the list of available formats and layouts returned by gvplugin_list() is empty. Added a prompt to configure plugins with dot -c if the response string was short (<2 chars) otherwise retained previous behavior - it should only show a list of formats and layouts if the list is not too short (not "empty").

There's minor churn from replacing tabs with spaces and adjusting indentation and bracing. I marked this as WIP because I'm not sure if prompting the user to run dot -c is a general solution or specific only to dot.

Pointers to project coding standards, etc. cheerfully accepted. I'm not the world's most confident C coder so I tend toward being more explicit and asking for review and comment.

This change was prompted by #1746 (closed) and could use some extended testing.