USGS SOAP API deprecated

From [the USGS Python client docs](http://kapadia.github.io/usgs/):
> As of May 15, 2017 the USGS has deprecated their SOAP API. This library has been updated to use their JSON API. As a result most, if not all, responses through this client will have a different structure compared to the older SOAP version.

This is most probably the reason why the error message could not be parsed in #233 (needs to be fixed in the [`usgs_eros`](https://github.com/developmentseed/landsat-util/blob/362d5b046f8c33112652b0a167e96332e9da4630/landsat/downloader.py#L96) function) - but not the reason for the error.

Start with landsat/downloader.py at the usgs_eros function referenced in the issue, then review the linked USGS Python client documentation and issue #233. Determine how the deprecated SOAP-to-JSON change affects the error response, and consider the work complete when the client handles that response without the parsing failure.

Written by the indexing model from the issue text.