all package list /seed parameter must be check before starting download of package

[alok]

a) if i select a package that is not available say xyz (i.e --seed zyz,.....)
rootstock starts downloading other packages
from alphabetical order and only reports error while retrieving xyz.
This consumes extensive time and bandwidth which eventually fails.
Instead rootstock should check availability/validity of each see parameter
and display error message very start of downloading of packages.
This would speed of rootstock build process in case user has selected
wrong package

b)similry If i select a package say ssh-server in --seed parameter ,it only gives
message that either of two packages i.e lsh-server or open-ssh-server should be
given as seed parameter when its actually getting this package.
Again this too eventually fails.

In short package dependencies,validity should be check before getting of packages.