internal/restic: Fix ID.UnmarshalJSON, ParseID
internal/restic: Fix ID.UnmarshalJSON, ParseID

ID.UnmarshalJSON accepted non-JSON input with ' as the string delimiter.
Also, the error message for non-hex input was less informative than it
could be and it performed too many checks.

Changed ParseID to keep the error messages consistent.

restic/restic,restic/restic