vm: Save current active exception on opening new try block.

[pfalcon]

Required to reraise correct exceptions in except block, regardless if more
try blocks with active exceptions happen in the same except block.

P.S. This "automagic reraise" appears to be quite wasteful feature of Python

• we need to save pending exception just in case it might be reraised.
  Instead, programmer could explcitly capture exception to a variable using
  "except ... as var", and reraise that. So, consider disabling argless raise
  support as an optimization.

[pfalcon]

This resolves (in one way) issues discussed in #379 (comment)

[dpgeorge]

Now that the previous exception is stored in this stack structure, would it not be possible to use that entry for the raise-with-no-args byte code? Ie instead of accessing nlr.ret_val and hoping it's the last exception raised, use exc_sp->exc_prev (or maybe exc_sp[-1] or something like that)?

[pfalcon]

Well, no hoping any more - 0c904df just restores it ;-). Using value directly on exc_stack probably won't work, because it's previous value and it might not exist. Probably ;-).

[dpgeorge]

See my recent commits: it can be done, and it's better to be done :)

[pfalcon]

it can be done, and it's better to be done :)

Cool, seems to have fixed #392 too ;-).