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.

lib/api/api_test.go

@@ -543,7 +543,7 @@ func startHTTP(cfg *mockedConfig) (string, *suture.Supervisor, error) {
 	tcpAddr, err := net.ResolveTCPAddr("tcp", addr)
 	if err != nil {
 		supervisor.Stop()
-		return "", nil, fmt.Errorf("Weird address from API service: %v", err)
+		return "", nil, fmt.Errorf("weird address from API service: %w", err)
 	}
 
 	host, _, _ := net.SplitHostPort(cfg.gui.RawAddress)