WebDriver: Introduce WebDriver::ErrorCode enum

To avoid having to duplicate error text and http codes over and over,
and potentially make mistakes, let's put them all in one place.

Userland/Services/WebDriver/CMakeLists.txt

@@ -8,6 +8,7 @@ set(SOURCES
     Client.cpp
     Session.cpp
     TimeoutsConfiguration.cpp
+    WebDriverError.cpp
     main.cpp
     )