bezier: properly complain when number of colors is bad

Maybe I should've generalized the 5 case into a bisecting or windowed implementation, but I am too lazy to actually test that.

Fixes #245.

src/generator/bezier.js

@@ -42,6 +42,8 @@ const bezier = function(colors) {
                 return I1((t-0.5)*2);
             }
         };
+    } else {
+        throw new RangeError("Bezier can only accept between 2 to 5 colors; got `${colors.length}`.")
     }
     return I;
 };