fix: improve error message when missing tags[browser]

[stdll00]

Handle error when invalid tags provided in py_web_test_suite and go_web_test_suite.
If tags[browser_name] is missing, below erorr will be raised.

        File ".../.cache/bazel/53d5c478a9b654e0091a6804f2465ac0/external/io_bazel_rules_webtesting/web/web.bzl", line 114, column 22, in _get_kwargs
                if not out_kwargs["tags"]:
Error: key "tags" not found in dictionary

After apply this PR, error message improved.

Error in fail: expeted "default" or "chromium-local" dict key in tags argument.

---

Example patch to reproduce error. (You can git apply to apply)
After apply this patch, run bazel build //testing/web:webtest_test_chromium-local to reproduce error.

diff --git a/testing/web/BUILD.bazel b/testing/web/BUILD.bazel
index 8d53025..2bea226 100644
--- a/testing/web/BUILD.bazel
+++ b/testing/web/BUILD.bazel
@@ -39,9 +39,6 @@ py_web_test_suite(
         "//browsers/sauce:chrome-win10-connect",
     ],
     tags = {
-        "chromium-local": [
-            "native",
-        ],
         "chrome-win10": [
             "exclusive",
             "sauce",