Make error message more descriptive and run informal tests

In the Draggable class, replace the error message in the definition of setEventDetector() and setEventResponder() with a template literal that includes the erroneous user input, so that it's easier for the user to see which part of their code is causing the problem.

Also run a few informal tests of the setters for both detectors and responders by adapting and executing the draggable line demo in pull request #9, including tests with and without erroneous input for the type parameter. Test coverage is not complete but is sufficient for the prototype.

Author: GregStanton

mathemagical-prototype.js

@@ -658,7 +658,7 @@ class Draggable {
           this.mousePressedPair[0] = detector;
           break;
         default:
-          throw new Error('Event type not currently supported. Please check docs and check for typos.');
+          throw new Error(`Event type ${type} not currently supported. Please check docs and check for typos.`);
       }
     }
     catch (error) {
@@ -685,7 +685,7 @@ class Draggable {
           this.mousePressedPair[1] = responder;
           break;
         default:
-          throw new Error('Event type not currently supported. Please check docs and check for typos.');
+          throw new Error(`Event type ${type} not currently supported. Please check docs and check for typos.`);
       }
     }
     catch (error) {