Improve errors in UI

[mkretz]

Currently, errors are mostly logged with log.Fatal. We need to integrate them in the UI mechanism properly using an error message.

This task is part of the ongoing decoupling of the checking code from the flow and user interaction code. The basic idea is that every function that currently exists with log.Fatal(...) will return an error instead and the CLI will exit in an orderly manner while logging the fatal error before exit.

High level goals of the refactoring:

• standard error flow is used
• checks are more contained (library like) and could be used in a different context (i.e. from a service)

[qasimwarraich]

Some progress has been made w/ 0a6d36f.