Better error response on main window timeout

Return a clear error response when loading the main window fails within
the page load timeout. Fixes #89.

src/FlaUI.WebDriver/Session.cs

@@ -20,7 +20,12 @@ public Session(Application? app, bool isAppOwnedBySession)
             if (app != null)
             {
                 // We have to capture the initial window handle to be able to keep it stable
-                CurrentWindowWithHandle = GetOrAddKnownWindow(app.GetMainWindow(Automation, PageLoadTimeout));
+                var mainWindow = app.GetMainWindow(Automation, PageLoadTimeout);
+                if (mainWindow == null)
+                {
+                    throw WebDriverResponseException.Timeout($"Could not get the main window of the app within the page load timeout (${PageLoadTimeout.TotalMilliseconds}ms)");
+                }
+                CurrentWindowWithHandle = GetOrAddKnownWindow(mainWindow);
             }
         }
 

src/FlaUI.WebDriver/WebDriverResponseException.cs

@@ -24,5 +24,7 @@ private WebDriverResponseException(string message, string errorCode, int statusC
         public static WebDriverResponseException WindowNotFoundByHandle(string windowHandle) => new WebDriverResponseException($"No window found with handle '{windowHandle}'", "no such window", 404);
 
         public static WebDriverResponseException NoWindowsOpenForSession() => new WebDriverResponseException($"No windows are open for the current session", "no such window", 404);
+
+        public static WebDriverResponseException Timeout(string message) => new WebDriverResponseException($"Timeout: ${message}", "timeout", 500);
     }
 }