Introduce/improve check_js_errors and improve test_no_implicit_target (#874)

Until now, we didn't have a nice way to check that we expect a specific JS error in the web page.
This PR improves check_js_errors() so that now you can pass a list of error messages that you expect.
It is tricky because we need to handle (and test!) all various combinations of cases:

- errors expected and found / expected but not found
- unexpected errors found / not found

Moreover, JS exceptions now are logged in the special category console.js_error, which means that the printed text is also available using e.g. self.console.js_error.text or self.console.all.text. However, this should never be required and it's preferred to use self.check_js_errors to check for exceptions. This fixes #795 .

Finally, use the new logic to improve test_no_implicit_target.

pyscriptjs/tests/integration/test_zz_examples.py

@@ -109,6 +109,7 @@ class TestExamples(PyScriptTest):
         assert self.page.title() == "Bokeh Example"
         wait_for_render(self.page, "*", '<div.*?class=\\"bk\\".*?>')
 
+    @pytest.mark.skip("flaky, see issue 759")
     def test_d3(self):
         self.goto("examples/d3.html")
         self.wait_for_pyscript()