Don't pass every error to the client

[FoseFx]

Is your feature request related to a problem? Please describe:

For what I can see the dapr SDK simply responds with err.Error() in case we return an error.
As the content type is text/plain we limit ourselves in the richness of errors we can provide.
We can not add translations or additional error codes or alike.

Describe the solution you'd like

Return a simple json object.

{
  "error": {
    "message": "..."
  }
}

We can extend it later if we need to.

Additional Comments

We should also update the content-type header to application/json and probably need to write middleware for this.

[MaxSchaefer]

We are moved to gRPC, but still our errors are quite "transparent".