lam: fail on I/O errors

I/O errors should be reported; however lam currently does not
disambiguate between EOF because end-of-file was reached and EOF because
an I/O error occurred.
This commit changes lam to exit with EX_IOERR when an I/O error occurs.

Reviewed by: imp, allanjude
Sponsored by: Apple Inc.
Differential Revision: https://reviews.freebsd.org/D45437