cli: make the command line parser's errors more informative.

Previously, the cli parser was a little erratic in what errors it
reported and would fail silently in many cases (for example, when no
argument was passed to an integer option). This was particularly
annoying as the user could not (easily) tell whether the command
failed or just there were no search results.

This patch tries to make the handling consistent and return a helpful
error message in all cases.