error.c: don't return value for void function

It is invalid to return a value from a function
returning void.

[C99 6.8.6.4 says "A return statement with an expression shall not
appear in a function whose return type is void" but gcc 4.6.3 with QEMU
compile flags does not complain.  It's still worth fixing this.  Stefan]

Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
