*: make utils.StatusError use struct values rather than pointers

This patch modifies the way utils.StatusError is used so that the
struct value (rather than a pointer to the struct value) implements
the error interface. This makes garbage collection easier for the
Go compiler and makes more sense than using pointers (as they are
treated as immutable within Docker).

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)