Error handling

[joepie91]

bhttp should allow for specifying a list or range of 'unacceptable' response codes (eg. 4xx, 5xx) that should be treated and thrown as errors.

• Allow specifying an array of 'error codes'.
• Consider what the default for this should be.
• Investigate the best API for specifying error code ranges. Perhaps just supply a few default lists, or allow predicates?
• Add an option for draining the response stream when errors occur (maybe this should be default and opt-out, like for keepRedirectResponses?)
• Investigate how to deal with encodeJSON for error responses; not all APIs consistently return JSON in case of errors (eg. Tahoe-LAFS, which can return plain text), and perhaps bhttp should be more lenient on this.