MM-10232, MM-10259: Improve error handling from invalid json (#8668)

* MM-10232: improve error handling from malformed slash command responses

Switch to json.Unmarshal, which doesn't obscure JSON parse failures like
json.Decode. The latter is primarily designed for streams of JSON, not
necessarily unmarshalling just a single object.

* rework HumanizedJsonError to expose Line and Character discretely

* MM-10259: pinpoint line and character where json config error occurs

* tweak HumanizeJsonError to accept err first

model/command_response.go

@@ -8,6 +8,8 @@ import (
 	"io"
 	"io/ioutil"
 	"strings"
+
+	"github.com/mattermost/mattermost-server/utils/jsonutils"
 )
 
 const (
@@ -31,14 +33,14 @@ func (o *CommandResponse) ToJson() string {
 	return string(b)
 }
 
-func CommandResponseFromHTTPBody(contentType string, body io.Reader) *CommandResponse {
+func CommandResponseFromHTTPBody(contentType string, body io.Reader) (*CommandResponse, error) {
 	if strings.TrimSpace(strings.Split(contentType, ";")[0]) == "application/json" {
 		return CommandResponseFromJson(body)
 	}
 	if b, err := ioutil.ReadAll(body); err == nil {
-		return CommandResponseFromPlainText(string(b))
+		return CommandResponseFromPlainText(string(b)), nil
 	}
-	return nil
+	return nil, nil
 }
 
 func CommandResponseFromPlainText(text string) *CommandResponse {
@@ -47,15 +49,19 @@ func CommandResponseFromPlainText(text string) *CommandResponse {
 	}
 }
 
-func CommandResponseFromJson(data io.Reader) *CommandResponse {
-	decoder := json.NewDecoder(data)
-	var o CommandResponse
+func CommandResponseFromJson(data io.Reader) (*CommandResponse, error) {
+	b, err := ioutil.ReadAll(data)
+	if err != nil {
+		return nil, err
+	}
 
-	if err := decoder.Decode(&o); err != nil {
-		return nil
+	var o CommandResponse
+	err = json.Unmarshal(b, &o)
+	if err != nil {
+		return nil, jsonutils.HumanizeJsonError(err, b)
 	}
 
 	o.Attachments = StringifySlackFieldValue(o.Attachments)
 
-	return &o
+	return &o, nil
 }