Let error codes start out at index 1.

When the error code facility was simplified one year ago in commit
a775408 (Simplify the error subsystem, get rid of error2.[ch]) we
introduced a simple enum for all error codes, but missed the fact
that this approach assigns the first error code the value zero.

Hence, when the AAC decoder returns this error code, it will not be
recognized as an error because we generally check for negative return
values only.

This patch introduces an unused dummy error code so that the real
error codes start at index 1.