Environment variable errors are not very clear

[PortableProgrammer]

Currently, when a required environment variable is not present, the only message returned is __main__ WARNING: Failed to find all environment variables!. This message is terribly unhelpful. We should be specifying which variable has the problem, and what the problem is. We do note when invalid values are found in variables, causing a reversion to default, so we should follow this pattern for required variables.

[PortableProgrammer]

With the additions of #47, #48, #49, #50, it's looking more and more like #35 is the correct answer here, and this should be closed / rolled in to the larger refactoring.