Assign getLayer template error message; catch and log error in addLayer

[dbauszus-glx]

An error object can not be sent with the layer json.

The Error.message can be assigned to the layer json instead.

The addLayer() method can check for an err message and log this.

[simon-leech]

I pushed a commit to change layer.err into an array.
I did this as we also need to check on layer.templates inside the getLayer method.

Example configuration for testing

"templates":{ 
   "test_1": {
     "template": "path/path.json",
    },
    "test_2_template": {
    "template": "path/path.json",
    },
    ,
    "test_3_template": {
    "template": "path/path.json",
    }
}, 
"layers": {
  "test_1": {
  "templates": ["test_2_template", "test_3_template"] 
}
}

The example above, before my commit you would only see an error for the overall layer file path being wrong.
With this commit we now also check on the file path of anything in the layer.templates array, and error those too.