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...