main: show TaskException instead of raising them

a TaskException already wraps another exception; raising it gives a
hard-to-read multi-line message instead of showing the clean wrapped
original backtrace; for comparison, an asserton error in image.py line
253:

exception AssertionError at /home/chrysn/git/whipper/morituri/image/image.py:253: stop(): Track length was not found; look for earlier errors in debug log (set RIP_DEBUG=4)
Traceback (most recent call last):
  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 497, in _startWrap
    task.start(self)
  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 395, in start
    BaseMultiTask.start(self, runner)
  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 330, in start
    self.next()
  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 401, in next
    BaseMultiTask.next(self)
  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 351, in next
    self.stop()
  File "/home/chrysn/git/whipper/morituri/image/image.py", line 253, in stop
    assert taskk.length is not None, "Track length was not found; look for earlier errors in debug log (set RIP_DEBUG=4)"
AssertionError: Track length was not found; look for earlier errors in debug log (set RIP_DEBUG=4)

vs.

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/chrysn/git/whipper/morituri/rip/main.py", line 27, in main
    ret = c.parse(sys.argv[1:])
  File "/home/chrysn/git/whipper/morituri/rip/main.py", line 94, in parse
    logcommand.LogCommand.parse(self, argv)
  File "/home/chrysn/git/whipper/morituri/common/logcommand.py", line 62, in parse
    command.Command.parse(self, argv)
  File "/home/chrysn/git/whipper/morituri/extern/command/command.py", line 401, in parse
    return self.subCommands[command].parse(args[1:])
  File "/home/chrysn/git/whipper/morituri/common/logcommand.py", line 62, in parse
    command.Command.parse(self, argv)
  File "/home/chrysn/git/whipper/morituri/extern/command/command.py", line 401, in parse
    return self.subCommands[command].parse(args[1:])
  File "/home/chrysn/git/whipper/morituri/common/logcommand.py", line 62, in parse
    command.Command.parse(self, argv)
  File "/home/chrysn/git/whipper/morituri/extern/command/command.py", line 363, in parse
    ret = self.do(args)
  File "/home/chrysn/git/whipper/morituri/rip/image.py", line 184, in do
    cueImage.setup(runner)
  File "/home/chrysn/git/whipper/morituri/image/image.py", line 75, in setup
    runner.run(verify)
  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 490, in run
    raise TaskException(task.exception, message=msg)
morituri.extern.task.task.TaskException: (AssertionError('Track length was not found; look for earlier errors in debug log (set RIP_DEBUG=4)',), 'exception AssertionError at /home/chrysn/git/whipper/morituri/image/image.py:253: stop(): Track length was not found; look for earlier errors in debug log (set RIP_DEBUG=4)\nTraceback (most recent call last):\n  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 497, in _startWrap\n    task.start(self)\n  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 395, in start\n    BaseMultiTask.start(self, runner)\n  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 330, in start\n    self.next()\n  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 401, in next\n    BaseMultiTask.next(self)\n  File "/home/chrysn/git/whipper/morituri/extern/task/task.py", line 351, in next\n    self.stop()\n  File "/home/chrysn/git/whipper/morituri/image/image.py", line 253, in stop\n    assert taskk.length is not None, "Track length was not found; look for earlier errors in debug log (set RIP_DEBUG=4)"\nAssertionError: Track length was not found; look for earlier errors in debug log (set RIP_DEBUG=4)\n')

morituri/rip/main.py

@@ -45,7 +45,10 @@ def main():
                 'rip: error: Could not create encoded file.\n')
             return 255
 
-        raise
+        # in python3 we can instead do `raise e.exception` as that would show
+        # the exception's original context
+        sys.stderr.write(e.exceptionMessage)
+        return 255
     except command.CommandError, e:
         sys.stderr.write('rip: error: %s\n' % e.output)
         return e.status