APKINDEX not found: Only show in verbose log (#1250)

The message appears 20 or so times in a row when setting up a new
chroot. Right now we log it into the debug log (which is what
"pmbootstrap log" and "pmbootstrap --details-to-stdout" show). This
is annoying for normal pmbootstrap usage, and really clutters up the
Travis output.

This commit changes the log level to verbose, so it only shows up
when "-v" is passed to pmbootstrap.