Use meaningful error messages when interacting with Linode resources

I hope these simple changes save other people some debug time. Instead of raising nil and swallowing an exception these will now output a meaningful error message. I was getting the following error when using this via knife and passing in an image ID that no longer exists:

```
gems/fog-1.14.0/lib/fog/core/collection.rb:115:in `new': Initialization parameters must be an attributes hash, got NilClass nil (ArgumentError)
	from gems/fog-1.14.0/lib/fog/linode/models/compute/images.rb:15:in `get'
	from bundler/gems/knife-linode-02ecd558b064/lib/chef/knife/linode_server_create.rb:190:in `run'
	from gems/chef-11.4.2/lib/chef/knife.rb:460:in `run_with_pretty_exceptions'
	from gems/chef-11.4.2/lib/chef/knife.rb:173:in `run'
	from gems/chef-11.4.2/lib/chef/application/knife.rb:123:in `run'
	from gems/chef-11.4.2/bin/knife:25:in `<top (required)>'
	from bin/knife:23:in `load'
	from bin/knife:23:in `<main>'
```

Thanks!