Better feedback for invalid .env file

[kristianvld]

The node server should give better feedback on missing/invalid variables in the .env file. E.g. if you are missing FUSION_AUTH_SECRET then you get a message containing TypeError: "secret" is required, but not mentioning the environment variable in question. There should also be some connection checks to to mongodb and fusion auth. At the moment you get a TypeError: "secret" is required error message when the mongodb connection is not successful.