build: 0.4.18-1
doc: deprecation policy.
py: detailed error reporting.

Have a string field too as the simplest way to tell what failed. Currently
only used for HY_E_IO error.

Related: RhBug:1117789
Hide hy_err_str from errno.h

Nobody needs to see it..
hy_err_str: it's best to make it static.

Consulted with <pmachata@redhat.com>.