Make some errors when running tests more friendly.

"Don't know what to do about" is not a helpful error.

* build/run_tests.py
  (TestHarness._run_c_test): Specify what we were testing the executable flag,
    prefix the error with a newline so it doesn't run into the test name,
    and simplify the code some.
  (TestHarness._run_py_test): Specify there was an error while loading the
    test, prefix error with newline like above, and print a traceback of
    the exception.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1558962 13f79535-47bb-0310-9956-ffa450edef68