Malformed rpc calls terminate connection

[Jeff Pihach]

When making an RPC call to log in on the Admin facade I was incorrectly sending the `version` as a string instead of an integer. Juju would then terminate the connection immediately instead of returning an error.

This took a while to track down, it would be nice if it returned an error for malformed requests.

```
--> {"type":"Admin","request":"Login","version":"3","params":{"maracoons":[]},"request-id":1}
close: 1006 Connection dropped by remote peer
```

[Pen Gale (pengale)]

Agree that Juju should be more helpful for errors like this.

Added the "bitesize" tag, as I believe that this should be pretty straightforward to add.

[Canonical Juju QA Bot (juju-qa-bot)]

This bug has not been updated in 2 years, so we're marking it Low importance. If you believe this is incorrect, please update the importance.