Better error handling in pkg add
Better error handling in pkg add

In case of ENOENT it only print the appropriated error message and advice the
user to use pkg install which is probable what he meant

Fixes #178
Reported by:	rwatson

junovitch/pkg,junovitch/pkg,khorben/pkg,khorben/pkg,Open343/pkg,khorben/pkg,en90/pkg,skoef/pkg,Open343/pkg,en90/pkg,skoef/pkg