BUG: Propagate error when assigning an instance to an array item.

Prior to this fix, numpy would swallow an error that may occur when
an instance of an old-style class would be used to set an item in an
incompatible array.  This would result in a SystemError.

Fixes #2276.
Merge pull request #4117 from abalkin/gh-2276

BUG: Propagate error when assigning an instance to an array item.