strerror: add missing ERFKILL and EHWPOISON.

There's little value to having strings for these, but we may as well
for consistency. There's no ELASTERRNO constant we can use to check
whether we've fallen behind again, but these were added in 2009 and
2011 respectively, so we're not likely to have to deal with any more
new errno values any time soon...

Bug: http://b/117755069
Test: ran tests
Change-Id: I5b7155c22cad92c2fe03a0b4c7cae415c37fa186