Missing Credentials exceptions barely understandable

[MarvinSchramm]

What problem are you trying to solve

When starting a service where no credentials have been provided the error message is not very helpfull

Current Behavior

Currently the message is relatively cryptic.
ApplicationCredentialsException: Unable to load credentials:'~/.slack/credentials' not all values are set

Possible Improvement

Make text human readable and provide example
and link to docs in error