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").

This is essentially !1442 (closed) but maintaining source code style to simplify review. Whitespace and commenting were revised based on comments - thanks for the feedback.