Warn when sending only HTML to users wanting only plain-text

If I use the Send an email feature to send to a particular contact or selected group of contacts, people may assume incorrectly that providing either HTML or plain-text will come through fine for their recipients given their experience of choosing either one or the other in most email clients. However, providing only HTML content to be sent to recipients marked to receive only plain text, or providing only plain text content for recipients marked to receive only HTML message leads to empty messages being sent out.

This is a usability issue that I just noticed but that is not important enough for me to provide a fix for. I'm not sure where these issues are supposed to go - here as issues or as unscheduled work in the wiki. When I provide them I mean them as a service to the community even if I'm not in a position to do the fix (in this case admittedly fairly trivial). If these sorts of things will be just be closed if I don't provide a patch, then I won't bother submitting them here.