Accumulate StdErr in a StringBuilder we can log

This allows us to consistently log these errors with all labels applied

changelogs/unreleased/320-improve-image-scaler-logs.yml

+---
+title: Improve logging for image scaler
+merge_request: 652
+author:
+type: other

internal/helper/helpers.go

@@ -37,10 +37,19 @@ func CaptureAndFail(w http.ResponseWriter, r *http.Request, err error, msg strin
 	LogError(r, err)
 }
 
+func CaptureAndFailWithFields(w http.ResponseWriter, r *http.Request, err error, msg string, code int, fields log.Fields) {
+	http.Error(w, msg, code)
+	LogErrorWithFields(r, err, fields)
+}
+
 func Fail500(w http.ResponseWriter, r *http.Request, err error) {
 	CaptureAndFail(w, r, err, "Internal server error", http.StatusInternalServerError)
 }
 
+func Fail500WithFields(w http.ResponseWriter, r *http.Request, err error, fields log.Fields) {
+	CaptureAndFailWithFields(w, r, err, "Internal server error", http.StatusInternalServerError, fields)
+}
+
 func RequestEntityTooLarge(w http.ResponseWriter, r *http.Request, err error) {
 	CaptureAndFail(w, r, err, "Request Entity Too Large", http.StatusRequestEntityTooLarge)
 }