nicer message when DistributionNotFound is raised

--HG--
branch : distribute
extra : rebase_source : b25209fbcd33b497cb92ed7d8f442403c4eaf152

CHANGES.txt

@@ -11,6 +11,7 @@ CHANGES
   Initial Patch by arfrever.
 * Issue 89: added a side bar with a download link to the doc.
 * Issue 86: fixed missing sentence in pkg_resources doc.
+* Added a nicer error message when a DistributionNotFound is raised.
 
 -----
 0.6.8

pkg_resources.py

@@ -528,7 +528,9 @@ class WorkingSet(object):
                         env = Environment(self.entries)
                     dist = best[req.key] = env.best_match(req, self, installer)
                     if dist is None:
-                        raise DistributionNotFound(req)  # XXX put more info here
+                        msg = ("The '%s' distribution was not found on this "
+                               "system, and is required by this application.")
+                        raise DistributionNotFound(msg % req)
                 to_activate.append(dist)
             if dist not in req:
                 # Oops, the "best" so far conflicts with a dependency