Display error messages from Assembly.scad to user

[laird]

If Assembly reports an error when asked for a preview, display the error instead of the preview.

Example errors: invalid inputs, incompatible options.

The communication channel would be the output log, with lines like:

Echo: ERRORMSG: [Knuckle width must be at least 55]
Echo: ERRORMSG: [This design does not have a Gauntlet.]
Echo: ERRORMSG: [Hand is too wide. Width must be at least 67 for length 84.], ERRORLINK: [wide.html]

The message would be displayed in place of the preview, and the link would be presented, if provided, for the user to get more information (in a new window).