Method to compare two screenshots. This is useful if you want to compare two screenshots rather than baseline <-> screenshot comparision.
For example to compare an element that is present in more than one page.

```
def test_element_all(self):
    if not self.save_baseline:
        self.assertCompareScreenshot('global.home', 'global.otherpage')
        self.assertCompareScreenshot('global.home', 'global.innerpage')
        self.assertCompareScreenshot('global.otherpage', 'global.innerpage')

    def test_element_home(self):
        self.driver.get(self.base_url + 'home.htm')
        # hier machen wir den screenshot mit dem CSS Selektor im 1. Parametern und den Screenshot namen als 2. Parametern
        self.assertScreenshot("#header", "global.home")

    def test_element_otherpage(self):
        self.driver.get(self.base_url + 'looks.detail_L040.htm')
        # hier machen wir den screenshot mit dem CSS Selektor im 1. Parametern und den Screenshot namen als 2. Parametern
        self.assertScreenshot("#header", "global.otherpage")

    def test_element_innerpage(self):
        self.driver.get(self.base_url + 'shoppingbag.full_G010.htm')
        # hier machen wir den screenshot mit dem CSS Selektor im 1. Parametern und den Screenshot namen als 2. Parametern
        self.assertScreenshot("#header", "global.innerpage")
```
Fixed error when opening file
Merge branch 'feature/assertCompareScreenshot'
Merge branch 'feature/imageopenerror'