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_py_repl.py

@@ -183,6 +183,8 @@ class TestPyRepl(PyScriptTest):
         tb_lines = err_pre.inner_text().splitlines()
         assert tb_lines[0] == "Traceback (most recent call last):"
         assert tb_lines[-1] == "Exception: this is an error"
+        #
+        self.check_py_errors("this is an error")
 
     @skip_worker("FIXME: display()")
     def test_multiple_repls(self):
@@ -228,6 +230,8 @@ class TestPyRepl(PyScriptTest):
         out_div = self.page.wait_for_selector("#py-internal-0-repl-output")
         assert "hello world" not in out_div.inner_text()
         assert "ZeroDivisionError" in out_div.inner_text()
+        #
+        self.check_py_errors("ZeroDivisionError")
 
     @skip_worker("FIXME: js.document")
     def test_hide_previous_error_after_successful_run(self):
@@ -253,6 +257,8 @@ class TestPyRepl(PyScriptTest):
         # test runner can be too fast, the line below should wait for output to change
         out_div = self.page.wait_for_selector("#py-internal-0-repl-output")
         assert out_div.inner_text() == "hello"
+        #
+        self.check_py_errors("this is an error")
 
     def test_output_attribute_does_not_exist(self):
         """