Clearer message for when there's a typo in the key

[webbertakken]

Context

In https://github.com/game-ci/steam-deploy/issues/23 it took us a while to find out there was simply a typo in the key.

Suggested solution

An error message with a descriptive text about the key missing would be great. Perhaps we should do this for all parameters that are required, or expected to be passed in.

Considered alternatives

None so far.

Additional details

None so far.