Exception Handling

Hello,
What is the intended behaviour of a plugin raising an exception?

e.g.

```
"plugins": {
"ext-plugin-pre-req": {
"allow_degradation": false,
"conf": [
{
"name": "netsuite-authenticator",
"value": "this is invalid json config"
}
]
}
}
```

```
def config(self, conf: Any) -> Any:
"""
Parse plugin configuration
:param conf:
:return:
"""
try:
conf_json = json.loads(conf)
except ValueError as e:
raise Exception("Configuration is invalid json")

return conf_json
```

```
2022-10-07 07:11:42,257 - INFO - request type:2, len:1792
2022-10-07 07:11:42,260 - ERROR - execute plugin `netsuite-authenticator` AnyError, ('Configuration is invalid json',)
2022-10-07 07:11:42,260 - INFO - response type:2, len:16
```

However **apisix ignores the plugin and continues with the request to the upstream**. This same occurs even if the exception is thrown in the filter() method.

I would prefer the ability to control this behaviour and I was expecting `allow_degredation` in apisix config to control the behaviour, but it seems this only apply to the plugin runner process itself, not errors from the plugin.

In some cases I also see the following errors immediately after the starting the plugin in dev mode due to the config token caching mechanism.

```
2022-10-07 07:37:13,104 - INFO - request type:2, len:1792
2022-10-07 07:37:13,106 - ERROR - token `1` cache acquisition failed
2022-10-07 07:37:13,106 - INFO - response type:2, len:16
```

Again, apisix ignores the plugin and continues with the request to the upstream, but I would rather it didn't.

I believe this is happening because the plugin runner is always returning a successful RPC response, but the "action" in the response isn't set to either stop or rewrite, so apisix just assumes the plugin didn't want to do anything.