Fix atypical error handling in `cmd` functions.

Closes #194

More changes to make the code base more Go idiomatic. Specifically, removed the logf utility function in favor of Printf. Additionally, where `cmd` functions were printing out an error and returning an error code, they now return the error directly, allowing main to print them before exiting with a non-zero code.
This seems funky and at least ought to go to stderr with some kind of prefix, IMO (maybe even bubble up, but that's a bigger refactor).