error log: clarify for reserving more resources than available

When trying to reserve more resources than available in the resources.conf,
actually print an intelligible error message: catch the nameless error from
solve() and fill in what was requested for solution.

Change-Id: Iba3707f1aaeb40a58c616c33af52a60c9a2e7e1f
diff --git a/selftest/resource_test.py b/selftest/resource_test.py
index 08f1fbf..c78485e 100755
--- a/selftest/resource_test.py
+++ b/selftest/resource_test.py
@@ -39,7 +39,7 @@
                      [2],
                      [0, 2] ]) 
     assert False
-except resource.NoResourceExn as e:
+except resource.NotSolvable as e:
     print(e)
 
 print('- test removing a Resources list from itself')