Provide real error messaging for functional test failures

Fixes theintern#178.

lib/wd.js

@@ -747,7 +747,10 @@ define([
 		if (/* not a private interface */ key.charAt(0) !== '_') {
 			PromisedWebDriver.prototype[key] = function () {
 				var self = this,
-					args = Array.prototype.slice.call(arguments, 0);
+					args = Array.prototype.slice.call(arguments, 0),
+					proxyError = new Error();
+
+				Error.captureStackTrace(proxyError);
 
 				this._lastPromise = when(this._lastPromise).then(function () {
 					var thisArg = self,
@@ -784,6 +787,32 @@ define([
 						self._context.push(lastReturnValue);
 					}
 					return lastReturnValue;
+				}, function (error) {
+					var command = key + '(' + args.map(function (value) {
+						if (typeof value === 'function') {
+							return value.toString();
+						}
+
+						return JSON.stringify(value);
+					}).join(', ') + ')';
+
+					if (error.message.indexOf('Failed to execute') === 0) {
+						throw error;
+					}
+
+					proxyError.message = 'Failed to execute ' + command + ': ' + error.message +
+						(error.cause && error.cause.value && error.cause.value.message ?
+							' ' + error.cause.value.message : '');
+
+					for (var k in error) {
+						if (k === 'message' || k === 'inspect' || k === 'stack') {
+							continue;
+						}
+
+						proxyError[k] = error[k];
+					}
+
+					throw proxyError;
 				});
 
 				return this;