Panic if the bee isn't found
Panic if the bee isn't found

Gives us a better error than dereferencing nil:

```
Fatal chain event: runtime error: invalid memory address or nil pointer dereference
```

versus:

```
Fatal chain event: Bee this-bee-name-is-wrong not registered
```

Mistyping the action's bee name in a recipe will cause this, like:

```
   "Chains":[
      {
         "Name": "Timer",
         "Description": "Alerts you at the right time",
         "Event":{
            "Bee":"cronbee",
            "Name":"time_event"
         },
         "Elements":[
            {
               "Action":{
                  "Bee":"this-bee-name-is-wrong",
                  "Name":"send",
                  "Options":[
                     {
                        "Name":"text",
                        "Value":"Cuckoo!"
                     }
                  ]
               }
            }
         ]
      }
   ]
```

muesli/beehive,muesli/beehive