Download non-200 response no longer swallowed

Based on exercism#794, we've decide to
error out and display API error messages.

This overlaps with cmd/submit, so this change extracts it for sharing.

cmd/cmd.go

@@ -81,6 +81,20 @@ func validateUserConfig(cfg *viper.Viper) error {
 	return nil
 }
 
+func decodedAPIErrorResponse(resp *http.Response) error {
+	var apiError struct {
+		Error struct {
+			Type    string `json:"type"`
+			Message string `json:"message"`
+		} `json:"error,omitempty"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&apiError); err != nil {
+		return fmt.Errorf("failed to parse error response - %s", err)
+	}
+
+	return fmt.Errorf(apiError.Error.Message)
+}
+
 // download is a download from the Exercism API.
 type download struct {
 	params *downloadParams
@@ -250,7 +264,7 @@ type solutionFileRequester struct {
 }
 
 // request requests a Solution file from the API, returning an HTTP response.
-// Non-200 responses and 0 Content-Length responses are swallowed, returning nil.
+// 0 Content-Length responses are swallowed, returning nil.
 func (s solutionFileRequester) request(filename string) (*http.Response, error) {
 	parsedURL, err := netURL.ParseRequestURI(
 		fmt.Sprintf("%s%s", s.download.Solution.FileDownloadBaseURL, filename))
@@ -269,8 +283,7 @@ func (s solutionFileRequester) request(filename string) (*http.Response, error)
 	}
 
 	if res.StatusCode != http.StatusOK {
-		// TODO: deal with it
-		return nil, nil
+		return nil, decodedAPIErrorResponse(res)
 	}
 	// Don't bother with empty files.
 	if res.Header.Get("Content-Length") == "0" {

cmd/submit.go

@@ -2,7 +2,6 @@ package cmd
 
 import (
 	"bytes"
-	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -252,12 +251,7 @@ func runSubmit(cfg config.Config, flags *pflag.FlagSet, args []string) error {
 	defer resp.Body.Close()
 
 	if resp.StatusCode == http.StatusBadRequest {
-		var jsonErrBody apiErrorMessage
-		if err := json.NewDecoder(resp.Body).Decode(&jsonErrBody); err != nil {
-			return fmt.Errorf("failed to parse error response - %s", err)
-		}
-
-		return fmt.Errorf(jsonErrBody.Error.Message)
+		return decodedAPIErrorResponse(resp)
 	}
 
 	bb := &bytes.Buffer{}
@@ -283,10 +277,3 @@ func runSubmit(cfg config.Config, flags *pflag.FlagSet, args []string) error {
 func init() {
 	RootCmd.AddCommand(submitCmd)
 }
-
-type apiErrorMessage struct {
-	Error struct {
-		Type    string `json:"type"`
-		Message string `json:"message"`
-	} `json:"error,omitempty"`
-}