Fix scrape failure logs

Before this change, logs would show like:

```
{...,"target":"http://localhost:8080/metrics","!BADKEY":"Get ..."}
```

After this change

```
{...,"msg":"Get ...","job_name":...,"target":...}
```

Signed-off-by: Simon Pasquier <spasquie@redhat.com>

scrape/scrape.go

@@ -1421,7 +1421,7 @@ func (sl *scrapeLoop) scrapeAndReport(last, appendTime time.Time, errc chan<- er
 		sl.l.Debug("Scrape failed", "err", scrapeErr)
 		sl.scrapeFailureLoggerMtx.RLock()
 		if sl.scrapeFailureLogger != nil {
-			sl.scrapeFailureLogger.Error("err", scrapeErr)
+			sl.scrapeFailureLogger.Error(scrapeErr.Error())
 		}
 		sl.scrapeFailureLoggerMtx.RUnlock()
 		if errc != nil {