Fix error code for unsupported locator strategy

W3C WebDriver spec indicates that when an unsupported locator strategy
is used in Find Element or Find Elements command, the implementation
should return "invalid argument" error. The existing JavaScript atoms
for these commands didn't specify an error code, resulting in the
default "unknown error". This commit fixes the error code.

Signed-off-by: Alexei Barantsev <barancev@gmail.com>

javascript/atoms/locators/locators.js

@@ -127,7 +127,8 @@ bot.locators.findElement = function(target, opt_root) {
       return strategy.single(target[key], root);
     }
   }
-  throw Error('Unsupported locator strategy: ' + key);
+  throw new bot.Error(bot.ErrorCode.INVALID_ARGUMENT,
+                      'Unsupported locator strategy: ' + key);
 };
 
 
@@ -154,5 +155,6 @@ bot.locators.findElements = function(target, opt_root) {
       return strategy.many(target[key], root);
     }
   }
-  throw Error('Unsupported locator strategy: ' + key);
+  throw new bot.Error(bot.ErrorCode.INVALID_ARGUMENT,
+                      'Unsupported locator strategy: ' + key);
 };