Make sure that tests fail in case there is an unhandled Python error (#1456)

Before this PR, the following test passed:

    def test_pyscript_hello(self):
        self.pyscript_run(
            """
            <script type="py">
                raise Exception("hello")
            </script>
            """)

What happens is that we intercept the Python exception and display a nice banner on the DOM, but the test itself passes. This is error prone: if we have Python exceptions on the page, the test should fail by default, and we should have a way to silence it in case those exceptions are expected.

This PR treats Python errors as we treat JS errors: unhandled exceptions cause the test to fail, but you can silence them by calling self.check_py_errors(), exactly as you can call self.check_js_errors().

pyscriptjs/tests/integration/test_event_handling.py

@@ -191,4 +191,5 @@ class TestEventHandler(PyScriptTest):
                 f"match banner text '{banner_text}'"
             )
 
-        assert any(msg in line for line in self.console.error.lines)
+        assert msg in self.console.error.lines[-1]
+        self.check_py_errors(msg)