Propagate original error in RuntimeError.cause (#1)

src/errors.js

@@ -1,6 +1,7 @@
-export function RuntimeError(message, input) {
+export function RuntimeError(message, input, cause = undefined) {
   this.message = message + "";
   this.input = input;
+  this.cause = cause;
 }
 
 RuntimeError.prototype = Object.create(Error.prototype);

src/variable.js

@@ -58,7 +58,7 @@ function variable_undefined() {
 function variable_rejector(variable) {
   return function(error) {
     if (error === variable_undefined) throw new RuntimeError(variable._name + " is not defined", variable._name);
-    if (error instanceof Error && error.message) throw new RuntimeError(error.message, variable._name);
+    if (error instanceof Error && error.message) throw new RuntimeError(error.message, variable._name, error);
     throw new RuntimeError(variable._name + " could not be resolved", variable._name);
   };
 }