Fireworks error messages now surface to callers

Patch Changes

• 550c127: Parse the object error envelope Fireworks actually returns ({"error":{"object","type","code","message"}}) instead of a bare string. The mismatched schema failed to parse, so error messages silently degraded to the HTTP reason phrase — Bad Request over HTTP/1.1, and an empty string over HTTP/2, which has no reason phrase. Fireworks' own message now reaches the caller, e.g. Model not found, inaccessible, and/or not deployed rather than Not Found.