Print better error messages when the dependency graph is broken.

parallel_emerge

Index: parallel_emerge
diff --git a/parallel_emerge b/parallel_emerge
index 4518575cf5e6fa3a298c277be587164404dd8e33..03684c2ed0d702685d95c46a0c33e53d46250b44 100755
--- a/parallel_emerge
+++ b/parallel_emerge
@@ -884,6 +884,20 @@ class DepGraphGenerator(object):
       if (this_pkg[merge_type] or pkg not in final_pkgs):
         return
 
+      if pkg not in deps_info:
+        emerge_cmd = "emerge"
+        if self.board:
+          emerge_cmd = "emerge-%s" % self.board
+        emerge_cmd += " -pe =%s %s" % (pkg, " ".join(emerge.cmdline_packages))
+        use_str = os.environ.get("USE")
+        if use_str:
+          emerge_cmd = 'USE="%s" %s' % (use_str, emerge_cmd)
+        print "ERROR: emerge has refused to update %s" % pkg
+        print "Are there impossible-to-satisfy constraints in the dependency"
+        print "graph? To debug the issue, try the following command:"
+        print "  %s" % emerge_cmd
+        sys.exit(1)
+
       # Mark this package as non-optional
       deps_info[pkg]["optional"] = False
       this_pkg[merge_type] = True