popen: refine popen_{read,write}_timeout errors

Popen backend errors should be meaningful for a user of the popen Lua
API, because otherwise we'll need to map backend errors into Lua API
errors. Those particular failures can't appear when the functions are
called from the Lua API, but it is good to keep all error messages in
one style.

Part of #4031

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>