compositor: fix return code from main()

compositor: fix return code from main()

There were a few cases of 'goto out' in main() that did not set ret to
EXIT_FAILURE. Shell failing to init is the one I hit when writing tests
for ivi-shell.

Rather than adding a few more 'ret = EXIT_FAILURE', make that the
default and remove the redundant assignments. When Weston exits
properly ec->exit_code will take care of the exit code.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>