Report browser exceptions (emscripten-core#8347)

This sends clientside browser exceptions to the test runner, and logs them out.

This may help debug failures on our CI, as before this an exception usually just silently hangs the test, and we need to try to reproduce it locally. Now at least we should get a stack trace.

tests/browser_error_reporting.js

@@ -0,0 +1,7 @@
+if (typeof window === 'object' && window) {
+  window.onerror = function(e) {
+    var xhr = new XMLHttpRequest();
+    xhr.open('GET', encodeURI('http://localhost:8888?exception=' + e));
+    xhr.send();
+  };
+}

tests/runner.py

@@ -1064,7 +1064,7 @@ def do_GET(self):
         self.send_header('Expires', '-1')
         self.end_headers()
         self.wfile.write(b'OK')
-      elif 'stdout=' in self.path or 'stderr=' in self.path or 'exception=' in self.path:
+      elif 'stdout=' in self.path or 'stderr=' in self.path:
         '''
           To get logging to the console from browser tests, add this to
           print/printErr/the exception handler in src/shell.html:
@@ -1075,6 +1075,8 @@ def do_GET(self):
         '''
         if DEBUG:
           print('[server logging:', urllib.unquote_plus(self.path), ']')
+      elif 'exception=' in self.path:
+        print('[client exception:', urllib.unquote_plus(self.path), ']')
       elif self.path == '/check':
         self.send_response(200)
         self.send_header('Content-type', 'text/html')
@@ -1333,6 +1335,7 @@ def btest(self, filename, expected=None, reference=None, force_c=False,
       self.reftest(path_from_root('tests', reference), manually_trigger=manually_trigger_reftest)
       if not manual_reference:
         args = args + ['--pre-js', 'reftest.js', '-s', 'GL_TESTING=1']
+    args += ['--pre-js', path_from_root('tests', 'browser_error_reporting.js')]
     all_args = [PYTHON, EMCC, '-s', 'IN_TEST_HARNESS=1', filepath, '-o', outfile] + args
     # print('all args:', all_args)
     try_delete(outfile)