Don't show "Screenshots: " in the error log when screenshots are disa…

[BorisOsipov]

When screenshots are disabled isn't added empty record in error log like this one:

Element not found {By.xpath: //div[@Class='dataTables_scrollBody']}
Expected: exist

Screenshot:
Timeout: 5 s.
Caused by: NoSuchElementException: no such element: Unable to locate element: {"method":"xpath","selector":"//div[@Class='dataTables_scrollBody']"}

[asolntsev]

@BorisOsipov Thank you for the PR!

[asolntsev]

@BorisOsipov I also added unit-test demonstrating the behaviour change:

com.codeborne.selenide.ex.ErrorMessagesTest

  @Test
  public void doesNotAddScreenshot_if_screenshotsAreDisabled() {
    Configuration.screenshots = false;

    String screenshot = ErrorMessages.screenshot();
    assertEquals("", screenshot);
    verify(screenshots, never()).takeScreenShot();
  }