Not adding an image to a new theme causes a traceback

[John Cegalis]

RC 3 This happens in Win7 and Kubuntu.

Create a new theme and choose "Image" from Background type. Do not add an image and proceed through the next steps to the end. When you click finish this traceback comes up. Expected result would be a notification that an image has not been selected either at the very end or when the person clicks the Next button leaving the image selection page.

Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.7/openlp/core/ui/themeform.py", line 627, in accept
    self.thememanager.saveTheme(self.theme, saveFrom, saveTo)
  File "/usr/lib/pymodules/python2.7/openlp/core/ui/thememanager.py", line 675, in saveTheme
    ImageSource.Theme, QtGui.QColor(theme.background_border_color))
  File "/usr/lib/pymodules/python2.7/openlp/core/lib/imagemanager.py", line 225, in updateImageBorder
    image = self._cache[(path, source)]
KeyError: (u'', 2)

[Phill (phill-ridout)]

Same bug as #1039221