Print exception type for py file exceptions, and perhaps traceback

[jsbain]

E.g. Something like

    except Exception as e:
        err_msg = '%s: %s (%s)\n' % (str(e.type), str(e), sys.exc_value)

In exec_py_file()

Possibly, it might be good to also print the traceback, at least one or two levels.

[ywangd]

@jsbain
Exception message should be more useful now by showing repr(e).
Also added a config option, py_traceback to display full traceback. Default is 0. You can add this option to .stash_config and turn it on by setting it to 1, i.e. py_traceback=1.
See 1ca6e4f

Thanks for the suggestions!