Alex Dowad authored Apr 22, 2015 and Timo Teräs committed Apr 22, 2015

fetch_maperror() translates error codes returned by libfetch to our error
codes. Handle those in apk_error_str(), returning error messages which
advise the user of the most likely fix.

A custom error code, EAPKSTALEINDEX, has been added for cases where
retrieving a package fails due to a HTTP error 404 or similar.

[TimoT: add also EAPKBADURL, as well as organize a bit better where the
EAPKSTALEINDEX is generated]