fix APIError to always report errors

[elv-gilles]

Command swagger generate client is generating client code looking like this:

// ReadResponse reads a server response into the received o.
func (o *DownloadQPartReader) ReadResponse(response runtime.ClientResponse, consumer runtime.Consumer) (interface{}, error) {


	readResponseError := func(err error) error {
		if response.Code()/100 == 2 {
			// that's a real error reading the response,
			// we might as well discard the response code ...
			return runtime.NewAPIError("read response", err, response.Code())
		}
		// now we're reading a potential error.. but if none don't say it's an error
		if err == io.EOF {
			err = nil
		}
		return runtime.NewAPIError("read response", err, response.Code())
	}

	switch response.Code() {

	case 200:
...

With errors that do not implement json marshalling the original error is wipped of the output:

cause [read response (status 200): {}]

After applying the patch:

cause [read response (status 200): 'write /path/to/my/file: no space left on device']

[codecov]

Codecov Report

Merging #243 (87042d5) into master (6df43e7) will increase coverage by 0.21%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
+ Coverage   79.94%   80.16%   +0.21%     
==========================================
  Files          42       42              
  Lines        3181     3186       +5     
==========================================
+ Hits         2543     2554      +11     
+ Misses        526      519       -7     
- Partials      112      113       +1     

Impacted Files	Coverage Δ
client_response.go	46.42% <50.00%> (+37.73%)	⬆️

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