prompt: fix missing file errors in zsh

zsh seems to have a bug while redirecting the stderr of the 'read'
command:

 % read foo 2> /dev/null < foo
 zsh: no such file or directory: foo

Which causes errors to be displayed when certain files are missing.
Let's add a convenience function to manually check if the file is
readable before calling "read".

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>