r14897: Do more error checking of tdb function returns and raise IOError or

r14897: Do more error checking of tdb function returns and raise IOError or
KeyError exceptions as appropriate.

Add a close() function to the wrapper as we can't rely on the
Python garbage collector destroying the object and closing the tdb file
at any particular time.
(This used to be commit a40d6c792257315d1eac955718db5ec1df7e07bb)