saveTiff, savePNG... raise a strange exception if folder of filename does not exist

[M. Gronle]

RuntimeError: cv: inwrite exited with false! 

is raised if the folder of the new image does not exist. The folder has to be created before.

The exemplary command is:

itom.algorithms.saveTiff(img, filename="C:/temp/myimg.tiff", palette="gray16")

‌

This issue is related to https://bitbucket.org/itom/itom/issues/228/command-to-save-images-in-tiff-format

Solution: Provide a better error message for this case.

[M. Gronle]

fixes in commit 148d433