[trivial] tester.nim add more diagnostics

[timotheecour]

• getCurrentExceptionMsg() was added in this block:

except IOError:
  given.err = reCodeNotFound

to provide useful diagnostics when IOError occur; otherwise looking at logs is not helpful:

example from a failed travis job: https://travis-ci.org/nim-lang/Nim/jobs/416117242

with this PR we get the useful line "cannot open: nimcache/tests/pragmas/tnoreturn.nim_0d61f8370cad1d412f80b84d143e1257/.tests.pragmas.tnoreturn.c" :

PASS: tcopying.nim JS -d:nodejs -d:release                         (1.76057816 secs)
cannot open: nimcache/tests/pragmas/tnoreturn.nim_0d61f8370cad1d412f80b84d143e1257/.tests.pragmas.tnoreturn.c
FAIL: tnoreturn.nim C
Test "tests/pragmas/tnoreturn.nim" in category "pragmas"
Failure: reCodeNotFound
Expected:
Gotten:

without this PR, all we get is the enigmatic "Failure: reCodeNotFound"

• " failed: ", failed was added in test result output, because it's easier for readers to see how many test failed without having to do some math