IDUTILS: exit with code 0 when ID not present

[Leo Liu]

I renamed ID to ID-old and find that global exits with 0 anyway. This is
an error and I think it should exit with non-zero code.

--8<---------------cut here---------------start------------->8---
-*- mode: ggtags-global; default-directory: "/home/shared/sources/c/dogecoin/" 
-*-
Global started at Tue Dec 10 11:37:31

global -v --result=grep --path-style=shorter --idutils --regexp "fDbEnvInit"
/usr/local/unix/idutils/bin/lid: can't open 
`/home/shared/sources/c/dogecoin/ID': No such file or directory
object not found (using idutils index in '/home/shared/sources/c/dogecoin').

Global found 0 identifiers at Tue Dec 10 11:37:31
--8<---------------cut here---------------end--------------->8---

Now rename ID-old back to ID to look at another issue:

--8<---------------cut here---------------start------------->8---
global -v --result=grep --path-style=shorter --idutils --regexp "fDbEnvInit"
global: The path in the output of lid is assumed relative.
'/home/leo/sources/c/dogecoin/src/db.h:38:    bool fDbEnvInit;'

Global exited abnormally with code 1 at Tue Dec 10 11:44:12
--8<---------------cut here---------------end--------------->8---

I have reported this issue before but it is funny this time. The ID file
was created by gtags itself using --idutils and yet global cannot handle
it.

I will keep the ID file around. Let me know if you need further
information.

Leo