Better error message

Output the last error message in addition to the final conflict
set, and give a less technical description of the conflict set for the
user message.

An example for the expected output is:

> cabal install --dry-run DPM CurryDB DAV DSH Digit HList MagicHaskeller bamboo
cabal: Could not resolve dependencies:
trying: CurryDB-0.1.1.0 (user goal)
trying: base-4.8.0.0/installed-1b6... (dependency of CurryDB-0.1.1.0)
trying: unix-2.7.1.0/installed-e59... (dependency of process-1.2.3.0/installed-3b1...)
trying: directory-1.2.2.0/installed-b49... (dependency of process-1.2.3.0/installed-3b1...)
Dependency tree exhaustively searched.
I've had most trouble fulfilling the following goals: haskell98 (36850), base
(26359), directory (9599), HTF (5092), bamboo (3788), DPM (3452), unix (3208),
CurryDB (107)

cabal-install/Distribution/Solver/Modular/Log.hs

@@ -79,12 +79,14 @@ logToProgress mbj l = let
     go ms (Step _ ns) (Step x xs)           = Step x (go ms ns xs)
     go ms r           (Step x xs)           = Step x (go ms r  xs)
     go ms _           (Fail (exh, cs, cm))  = Fail $
-        "Could not resolve dependencies:\n" ++ case exh of
+        "Could not resolve dependencies:\n" ++
+        unlines (messages $ showMessages (L.foldr (\ v _ -> v `CS.member` cs) True) False ms) ++
+        case exh of
             Exhaustive ->
                 "Dependency tree exhaustively searched.\n" ++
-                "Final conflict set is: " ++ CS.showCSWithFrequency cm cs
+                "I've had most trouble fulfilling the following goals: "
+                ++ CS.showCSWithFrequency cm cs
             NotExhaustive ->
-                unlines (messages $ showMessages (L.foldr (\ v _ -> v `CS.member` cs) True) False ms) ++
                 "Backjump limit reached (" ++ currlimit mbj ++
                 "change with --max-backjumps or try to run with --reorder-goals).\n"
               where currlimit (Just n) = "currently " ++ show n ++ ", "