We now validate the parsed config.json and check for any omitted

fields. Go doesn't let us error on a missing field during

json.Unmarshall so we have to check for its 0 value (e.g., "" for

strings). We now return an error if any required field is missing. In

the case of a missing Port field, we default to 8888 and log a warning

message.
Additionally, all files specfied in config.json are resolve relative

to the path of the config file unless they are absolute paths.

Testing done:

Ran go fmt ./....

Ran go test ./....