Fix double-output of "Error: " label for command errors

This patch was authored and released by @gwynne.

Because CommandError.description includes the "Error: " label in the returned text, practically all formatted error output (especially such as found in vapor/toolbox) ends up printing Error: Error: ... for command errors. Since this table is redundant in the description of an Error anyway, we just remove it. The description of an Error is human-readable and therefore not parseable, and is thus not considered API, so this is considered a semver-patch change.