PEP 3134 gives reason to change it.  __context__ should be set from
whatever exception is "active" from the try/finally, thus it should be
the inner block, not the outer except block.

This flipping of behaviour, and the general ambiguity, is why I suggest
a syntax error.  "In the face of ambiguity, refuse the temptation to guess."

PEP 3134 has not been officially accepted, but many parts have be added
anyway.  Your cleanups pave the way for the last of it.  I suggest
asking on python-3000 for a pronouncement on the PEP.