Add retry block for HTTPError 5xx from Entrez.efetch()

[vincentdavis]

Add try > except > pause > retry block for HTTPError 5xx type errors for Entrez.efetch() batch example

[vincentdavis]

I think this is ready

[peterjc]

Sorry, didn't see those changes.

What I meant above was while we would use Bio._py3k inside Biopython to import the HTTPError, this is intended as a private interface and so should not be used in our user documentation. Thus the try/except would be the neatest solution.

[vincentdavis]

Ok, done, or at least in need of another review.

[peterjc]

Applied as the single commit e2ed045 with one minor tweak 48fdd80 - thanks :)