Hm, I think I'd rather something fail if the user chooses incompatible options rather than it adjusting things so that they work. There are two options for this:

1. Don't generate the source map and keep going
2. Fail the task fatally; don't even generate the dest file

Of these two options, I think the first makes the most sense. A good error message would be enough to let them know what they need to change to generate the source map that they want.

What do you think, @mzgoddard?