Handle TypeErrors during ENV assignment

This introduces a new error, ClimateControl::UnassignableValueError,
which is raised when attempting to assign a key/value to ENV which can't
be handled.

Closes #5
@anthonynavarre It'll be a `TypeError`, with the format of "no implicit conversion of TYPE into String". You'd see the same error if you attempt to assign a value in ENV outside of Climate Control too, although I agree that it's not necessarily obvious. Since it's dependent on the class, do you have any recommendations on how to better handle it?