strings: fix exit status if a file before the last one fails

[emaste]

Summary

Previously a command like "strings f1 f2 f3" reported the exit status based only on processing the last file.

As with GNU strings, report an error exit status if an error was encountered processing any of the files. While here simplify the exit status handling to just success (0) / failure (1).

[brooks]

LGTM

[kaiw]

patch looks good to me