Improved debugging for failed extension imports

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When failing to import an extension, it can fail in ways that we don't
get a good import error exception.

Specifically, I just ran into a case where we failed to import
_chk_map_pyx.pyx because it depended on zlib1.dll. And while that dll
was in the path, the permissions were set such that it could not be
loaded as a dll (read but not execute).

Under that circumstance, the failure message was simply:
0.812 failed to load extensions: ['DLL load failed: Access is denied.']

There would be a couple possible helpers here:

 1) Change the api to be:

- -def failed_to_load_extension(exception):
+def failed_to_load_extension(name, exception):

(or we could make name an optional second argument if we wanted
backwards compatibility).

 2) Change -Derror (or add a new flag) such that if an extension fails
to load, we either raise the import exception directly, or we give a
quick traceback of the failure.

I actually found the problem because while changing the interface, I got
a clear traceback with the code that was failing to import but had not
been updated yet... :)

Both would be fine for me. I think most cases of an extension failing to
import will include the module, so having to update all the locations is
probably overkill.

John
=:->

  affects bzr
  status confirmed
  importance medium
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