Allow symbol or string for building package.

This makes sense since functions such as package-build-checkout take a
symbol as argument.  Preferably package-build should use one of them all
over place and not mix.
Improve debug information when running process fail.
Default directory should end with slash.

We can only do that if the directory already exist.  But sometimes the
command that is to be run is what creates the directory, hence it does
not exist.  So we only end with slash if the directory exists.
Merge pull request #1432 from rejeep/misc-package-build-improvements

Misc package build improvements