Fix json error handling

[eguzki]

For JSON endpoints, when 3scale API returns http.StatusUnprocessableEntity response status code, the body contains the specific error format:

struct {                               
    Errors map[string][]string `json:"errors"`   
}                                              

For instance:

{"errors":{"system_name":["has already been taken"]}}

Current implementation was shadowing the actual error with decoding error as it was trying to parse with

map[string]string

For the generic error response, the body is just readed and not parsed. This breaks backwards compatibility for a given set of endpoints, resulting a slightly different error message than before. For example:
Before:

error calling 3scale system - reason: error - Your access token does not have the correct permissions - code: 403"

Now:

error calling 3scale system - reason: { "error": "Your access token does not have the correct permissions" } - code: 403

If this an inconvenience, the code may be improved to process specific cases. Which I do not think so, but asking for safety :)

[philipgough]

@eguzki - I don't see an issue with this change. The errors are unexported so users would not be expected to test against the error code regardless. I don't see it breaking anything we use internally and besides that, the api is alpha so happy to merge.