Give understandable error if configuration is faulty

[schemar]

As an admin
I want to get informed if the config is faulty
Because I need to know how to fix it

Given that I start phabulous
When an important config parameter is missing
Then the log should tell me exactly which one is missing
But instead I just get:
runtime error: invalid memory address or nil pointer dereference

See also #39 and #63.