Provide better diagnostics output

[evnu]

Failed test cases are currently (as of d79a879) reported in the usual stack-tracy way:

./doctest -pa ebin -pa apps/examples/ebin -source apps/examples/src/failing.erl
escript: exception error: no match of right hand side value no_fail
  in function  doctest_2:run/0 (, line 4)
  in call from doctest:'-run/1-lc$^0/1-0-'/1 (src/doctest.erl, line 33)
  in call from doctest:'-main/1-lc$^0/1-0-'/1 (src/doctest.erl, line 20)
  in call from escript:run/2 (escript.erl, line 757)
  in call from escript:start/1 (escript.erl, line 277)
  in call from init:start_em/1 
  in call from init:do_boot/3

When running doctest against multiple files, the output does not indicate which test actually failed:

./doctest -pa ebin -pa apps/examples/ebin -source apps/examples/src/ex1.erl apps/examples/src/failing_edgecase.erl apps/examples/src/failing.erl 
No problems with side effects
escript: exception error: no match of right hand side value no_fail
  in function  doctest_18:run/0 (, line 4)
  in call from doctest:'-run/1-lc$^0/1-0-'/1 (src/doctest.erl, line 33)
  in call from doctest:'-main/1-lc$^0/1-0-'/1 (src/doctest.erl, line 20)
  in call from doctest:'-main/1-lc$^0/1-0-'/1 (src/doctest.erl, line 20)
  in call from escript:run/2 (escript.erl, line 757)
  in call from escript:start/1 (escript.erl, line 277)
  in call from init:start_em/1 
  in call from init:do_boot/3

A better output would be something like this:

[success] apps/examples/src/ex1.er
[success] apps/examples/src/failing_edgecase.erl
[failed] apps/examples/src/failing.erl
escript: exception error: no match of right hand side value no_fail
  in function  doctest_18:run/0 (, line 4)

The stacktrace should be cropped to the important part (the actual test), and each file should be mentioned specifically.