Add link to Troubleshooting page to Live Preview error messages

This is for #7634 for Brackets Milestone 1.0

This is to append link to troubleshooting doc to live preview error message. I went ahead and added the link to all live preview error messages.

@njx I did not reword error message as called for in #7634 -- is this ok? If no, suggestions for rewording? 

Small error (the reason the Travis build failed): function should be `_appendTroubleshootingMessage`.