Include description for missing bundle prereqs

When throwing MissingBundlePrerequisiteException we
also include the short description, if available, of
each missing object.

This is the fix for the following issue:
http://code.google.com/p/egit/issues/detail?id=25



Change-Id: I5d45aec7873af76a12170d9a500626a7264f2c42
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>