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_02_output.py

@@ -95,17 +95,9 @@ class TestOutput(PyScriptTest):
         self.page.locator("text=Click me").click()
         text = self.page.text_content("body")
         assert "hello" not in text
-
-        # currently the test infrastructure doesn't allow to easily assert that
-        # js exceptions were raised this is a workaround but we need a better fix.
-        # Antonio promised to write it
-        assert len(self._page_errors) == 1
-        console_text = self._page_errors
-        assert (
+        self.check_js_errors(
             "Implicit target not allowed here. Please use display(..., target=...)"
-            in console_text[0].message
         )
-        self._page_errors = []
 
     def test_explicit_target_pyscript_tag(self):
         self.pyscript_run(