all: Tweak error creation (#6391)

- In the few places where we wrap errors, use the new Go 1.13 "%w"
  construction instead of %s or %v.

- Where we create errors with constant strings, consistently use
  errors.New and not fmt.Errorf.

- Remove capitalization from errors in the few places where we had that.

cmd/strelaypoolsrv/main.go

@@ -10,6 +10,7 @@ import (
 	"context"
 	"crypto/tls"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -494,7 +495,7 @@ func handleRelayTest(request request) {
 		if debug {
 			log.Println("Test for relay", request.relay, "failed")
 		}
-		request.result <- result{fmt.Errorf("connection test failed"), 0}
+		request.result <- result{errors.New("connection test failed"), 0}
 		return
 	}