Improve Dialyzer output for scan errors

To reproduce, create an empty file "foo.erl" and run dialyzer on it.
Results without this patch:

$ dialyzer --src /tmp/foo.erl
  Checking whether the PLT /Users/magnus/.dialyzer_r16_plt is up-to-date... yes
  Proceeding with analysis...
dialyzer: Analysis failed with error:
Could not scan the following file(s): [{"/tmp/foo.erl",
                                        ["/tmp/foo.erl:1: no module definition\n"]}]
Last messages in the log cache:
  Reading files and computing callgraph...

Results with this patch:

$ dialyzer --src /tmp/foo.erl
  Checking whether the PLT /Users/magnus/.dialyzer_r16_plt is up-to-date... yes
  Proceeding with analysis...
dialyzer: Analysis failed with error:
Could not scan the following file(s):
/tmp/foo.erl:1: no module definition

Last messages in the log cache:
  Reading files and computing callgraph...

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/dialyzer/src/dialyzer_analysis_callgraph.erl b/lib/dialyzer/src/dialyzer_analysis_callgraph.erl
index 86618a4915..ca6b403ac9 100644
--- a/lib/dialyzer/src/dialyzer_analysis_callgraph.erl
+++ b/lib/dialyzer/src/dialyzer_analysis_callgraph.erl
@@ -261,8 +261,8 @@ compile_and_store(Files, #analysis_state{codeserver = CServer,
                     dict:new(), NewFiles),
       check_for_duplicate_modules(ModDict);
     false ->
-      Msg = io_lib:format("Could not scan the following file(s): ~p",
-			  [lists:flatten(Failed)]),
+      Msg = io_lib:format("Could not scan the following file(s):~n~s",
+      			  [[Reason || {_Filename, Reason} <- Failed]]),
       exit({error, Msg})
   end,
   {T2, _} = statistics(runtime),