Throw if a stylesheet is asked for but not downloadable

lib/carto/renderer.js

@@ -152,6 +152,9 @@ carto.Renderer = function Renderer(env) {
                                 .on('complete', function(external) {
                                     m.Stylesheet[k] = external.path();
                                     next();
+                                })
+                                .on('err', function(err) {
+                                    throw 'Stylesheet "' + s +'" could not be loaded.';
                                 });
                         }
                     });