Replace IsUserRoot with CheckIfRoot. This allows all functions to use
the same error handling when a user is not root.

When looking up users in fscrypt, we often want to proceed even if the
requested uid doesn't appear to be a valid user on the system. This
mainly occurs when a user is deleted, but they still have a login
protector on disk.

Thus, GetUser() with a bad uid creates a fake user with a pretty
placeholder name. The corresponding call sites of util.EffectiveUser are
changed (often simplifying logic). Various documentation is updated and
typos are fixed.

This commit changes how we handle InvalidInput and System errors.
Instead of having formatting functions, the now just wrap a string and
should be created with fmt.Sprintf or similar. We also move all of the
error related code into a single place.

Finally, the utils package gets additional functionality with MinInt64
and GetUsername, and the UnderlyingError function gets better logging.

Note that this will break packages that depend on it. For instance,
metadata and crypto currently do not build. This is fixed in a later
commit.

Change-Id: I819e4d1970604456a5b4b6a7c86426f180a6d092