Bug 997285 part 1.  Add JS_GetErrorPrototype to JSAPI.  r=jorendorff
Bug 997285 part 2.  Add [ExceptionClass] support to WebIDL.  r=peterv
Bug 997285 part 3.  Make DOMException [ExceptionClass].  r=peterv,jorendorff
Bug 997285 part 4.  Drop the custom stringifier from DOMException in favor of the default one on Error.prototype.  r=peterv,robcee