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/src/pyconfig.ts

@@ -165,7 +165,15 @@ function parseConfig(configText: string, configType = "toml") {
         catch (err) {
             const errMessage: string = err.toString();
             showError(`<p>config supplied: ${configText} is an invalid TOML and cannot be parsed: ${errMessage}</p>`);
-            throw err;
+            // we cannot easily just "throw err" here, because for some reason
+            // playwright gets confused by it and cannot print it
+            // correctly. It is just displayed as an empty error.
+            // If you print err in JS, you get something like this:
+            //     n {message: '...', offset: 19, line: 2, column: 19}
+            // I think that 'n' is the minified name?
+            // The workaround is to re-wrap the message into SyntaxError(), so that
+            // it's correctly handled by playwright.
+            throw SyntaxError(errMessage);
         }
     }
     else if (configType === "json") {