Wallaby.Browser.visit/2 does not generate an error if server is unreachable

[ngeraedts]

Issue

If the page to be visited is unreachable, the valid/2 function does not raise an error, leading to some potentially misleading error messages down the code path.

Test Code & HTML

The following test case and error message were discovered because I had failed to follow instructions and set Phoenix's server: true in my config/test.exs file. PEBKAC. :)

Using the following test:

  test "view homepage", %{session: session} do
    cookies =
      session
      |> visit("/")
      |> set_cookie("foo", "bar")
      |> visit("/")
      |> Browser.cookies()
      |> hd()
    assert cookies["name"] == "foo"
    assert cookies["value"] == "bar"
  end

We get the following error message:

     ** (Wallaby.CookieError) The cookie you are trying to set has no domain.
     You're most likely seeing this error because you're trying to set a cookie before
     you have visited a page. You can fix this issue by calling `visit/1`
     before you call `set_cookie/3`.

It would be useful if the visit/2 call would generate an error indicating that the browser was unable to reach the desired page.