Handle the JSON parse error in HParams plugin (#5970)

## Motivation for features / changes
Currently, feeding badly formatted request data to
`tb.corp/experiment/<id>/data/plugin/hparams/experiment` endpoint can
result in 500s (see
[b/250791957](https://b.corp.google.com/issues/250791957)):

```
...
    raise ParseError('Failed to load JSON: {0}.'.format(str(e)))
google3.net.proto2.python.public.json_format.ParseError: Failed to load JSON: Expecting value: line 1 column 1 (char 0).
```
The 500s are confusing to the end users because they will see an
`Internal Server Error` page without any useful debugging information.
It also adds unnecessary noise to the logging/monitoring system which
makes it hard to found the real alerts.

## Technical description of changes
This CL catches the JSON ParseError, logs the request data for
debugging, and throws 400s.

## Screenshots of UI changes
   
<img width="1105" alt="image"
src="https://user-images.githubusercontent.com/88216042/195377616-d06b4bd8-0532-4b37-9f09-d6e7b8dc5885.png">

## Detailed steps to verify changes work correctly (as executed by you)
Open the browser and enter the hparams plugin endpoint of an experiment
that feeds the badly-formatted request data, e.g
`/experiment/1811564056169732114/data/plugin/hparams/experiment?request=blah`
and you should see the UI changes above rather than an `Internal Server
Error` page.

Author: qihach64

tensorboard/plugins/hparams/hparams_plugin.py

@@ -191,13 +191,18 @@ def _get_scalars_plugin(self):
 
 
 def _parse_request_argument(request, proto_class):
-    if request.method == "POST":
-        return json_format.Parse(request.data, proto_class())
-
-    # args.get() returns the request URI-unescaped.
-    request_json = request.args.get("request")
-    if request_json is None:
+    request_json = (
+        request.data
+        if request.method == "POST"
+        else request.args.get("request")
+    )
+    try:
+        return json_format.Parse(request_json, proto_class())
+    # if request_json is None, json_format.Parse will throw an AttributeError:
+    # 'NoneType' object has no attribute 'decode'.
+    except (AttributeError, json_format.ParseError) as e:
         raise error.HParamsError(
-            "Expected a JSON-formatted 'request' arg of type: %s" % proto_class
-        )
-    return json_format.Parse(request_json, proto_class())
+            "Expected a JSON-formatted request data of type: {}, but got {} ".format(
+                proto_class, request_json
+            )
+        ) from e