Generalize error return

[dbutenhof]

Currently pyesbulk returns an error summary tuple; this should at least be packaged into a cleaner named tuple. However, more importantly, error documents discovered when traversing the response generator are written to a file pointer as pretty-printed JSON, which is difficult for a caller to consume.

I'd like to repackage pyesbulk as an object, with the existing entry points being thin compatibility wrappers as in many other packages. Ideally, the object bulk methods would return a list of JSON dicts (or possibly a generator, in case there are many?), which the caller could consume more easily. (The compatibility wrapper would pretty-print and write to the file as it does now.)