import-checker.py: exit with code 0 if no error is detected...
import-checker.py: exit with code 0 if no error is detected
Before this patch, `import-checker.py` exits with non-0 code, if no
error is detected. This is unusual as Unix command.
This change may be a one of preparations for issue4677, because this
can avoid extra explanation about unusual exit code of
`import-checker.py` for third party tool developers.
