Bug 1326534 - Propagate stacktraces for WebDriver errors; r=automatedtester

When we currently create new WebDriver errors we throw away the stacktrace
generated by `WebDriverError`'s prototype, `Error`.  This change stores
the stacktrace, which will cause it to be serialised and returned to
the client.

This change is not as valuable as storing the stacktraces of internal
errors, but brings symmetry to our error handling and may be useful if
only to navigate to the source of an error.

MozReview-Commit-ID: LCFMwKxxcTp

--HG--
extra : rebase_source : 56947805f29000a64c2daef0fd774ea90330c09e

testing/marionette/error.js

@@ -211,7 +211,7 @@ error.fromJson = function (json) {
  *     to propagate.
  */
 this.WebDriverError = function (err) {
-  Error.call(this, err);
+  const proto = Error.call(this, err);
   this.name = "WebDriverError";
   this.status = "webdriver error";
 
@@ -220,6 +220,7 @@ this.WebDriverError = function (err) {
     this.stack = err.stack;
   } else {
     this.message = err;
+    this.stack = proto.stack;
   }
 };
 WebDriverError.prototype = Object.create(Error.prototype);

testing/marionette/test_error.js

@@ -81,12 +81,15 @@ add_test(function test_toJson() {
       /Unserialisable error type: [object Error]/);
 
   let e0 = new WebDriverError();
-  deepEqual({error: e0.status, message: "", stacktrace: ""},
-      error.toJson(e0));
+  let e0s = error.toJson(e0);
+  equal(e0s.error, "webdriver error");
+  equal(e0s.message, "");
+  equal(e0s.stacktrace, e0.stack);
 
   let e1 = new WebDriverError("a");
-  deepEqual({error: e1.status, message: "a", stacktrace: ""},
-      error.toJson(e1));
+  let e1s = error.toJson(e1);
+  equal(e1s.message, e1.message);
+  equal(e1s.stacktrace, e1.stack);
 
   let e2 = new JavaScriptError("first", "second", "third", "fourth");
   let e2s = error.toJson(e2);
@@ -107,10 +110,17 @@ add_test(function test_fromJson() {
   Assert.throws(() => error.fromJson({}),
       /Undeserialisable error type: undefined/);
 
+  // stacks will be different
   let e1 = new WebDriverError("1");
-  deepEqual(e1, error.fromJson({error: "webdriver error", message: "1"}));
+  let e1r = error.fromJson({error: "webdriver error", message: "1"});
+  equal(e1r.status, e1.status);
+  equal(e1r.message, e1.message);
+
+  // stacks will be different
   let e2 = new InvalidArgumentError("2");
-  deepEqual(e2, error.fromJson({error: "invalid argument", message: "2"}));
+  let e2r = error.fromJson({error: "invalid argument", message: "2"});
+  equal(e2r.status, e2.status);
+  equal(e2r.message, e2.message);
 
   let e3 = new JavaScriptError("first", "second", "third", "fourth");
   let e3s = error.toJson(e3);