AssertionError is not clear enough for bootstrap.py

[NicDumZ]

Hello!

An eventual error when spawning a child in bootstrap.py raises a cryptic AssertionError, mainly to due to linewrapping

Traceback ...:
  File "bootstrap/boostrap.py", line 112, in ?
      PYTHONPATH=
AssertionError

I believe that a nicer, friendly error message can be written, quite quickly :)
[such errors can happen, if python has an invalid install, for instance missing development headers]

As a quick note. Is there any reason why you are not using subprocess for the non-jython code path? Unless buildout is supposed to support python2.3, the code can probably be simplified.
I'm attaching a dummy patch to explain what I'm hinting at: I left the error message empty as I'm not able to detail the inners of this failing step.

Also, if subprocess is used everywhere, the quote() thing might not be necessary at all: I dont have the necessary platform to test this, but I suppose that subprocess escapes correctly arguments on windows, right?

Regards,