Address gosec findings

Mostly handling unhandled errors appropriately.
If logging to STDERR fails, we panic. Added #nosec
comments to findings we are OK with.

pkg/upstream/proxy.go

@@ -85,6 +85,9 @@ func NewProxyErrorHandler(errorTemplate *template.Template, proxyPrefix string)
 			Message:     "Error proxying to upstream server",
 			ProxyPrefix: proxyPrefix,
 		}
-		errorTemplate.Execute(rw, data)
+		err := errorTemplate.Execute(rw, data)
+		if err != nil {
+			http.Error(rw, "Internal Server Error", http.StatusInternalServerError)
+		}
 	}
 }