BUG: Fix error handling for image/mask mismatch

In `imageoperations.checkMask` and error is thrown if the image and mask do not align or contain tolerance errors. The error message is caught and parsed to provide users with a more insightful message providing potential solutions. However, this was done by evaluating the `message` attribute of the RunTimeError thrown by SimpleITK. This attribute is present in python 2, but not in python 3.
Instead, use `args[0]` attribute, which contains the message, and is present in both python 2 and 3.

CHANGES.rst

@@ -11,6 +11,7 @@ Bug fixes
 
 - Ensure PyKwalify has a log handler, which is needed when parameter file validation fails.
   (`#309 <https://github.com/Radiomics/pyradiomics/pull/309>`_)
+- Fix bug in error handling in :py:func:`~radiomics.imageoperations.checkMask` (compatibility issue between python 2 and 3).
 
 Tests
 #####

radiomics/imageoperations.py

@@ -234,11 +234,11 @@ def checkMask(imageNode, maskNode, **kwargs):
   except RuntimeError as e:
     # If correctMask = True, try to resample the mask to the image geometry, otherwise return None ("fail")
     if not kwargs.get('correctMask', False):
-      if "Both images for LabelStatisticsImageFilter don't match type or dimension!" in e.message:
+      if "Both images for LabelStatisticsImageFilter don't match type or dimension!" in e.args[0]:
         logger.error('Image/Mask datatype or size mismatch. Potential solution: enable correctMask, see '
                      'Documentation:Usage:Customizing the Extraction:Settings:correctMask for more information')
         logger.debug('Additional information on error.', exc_info=True)
-      elif "Inputs do not occupy the same physical space!" in e.message:
+      elif "Inputs do not occupy the same physical space!" in e.args[0]:
         logger.error('Image/Mask geometry mismatch. Potential solution: increase tolerance using geometryTolerance, '
                      'see Documentation:Usage:Customizing the Extraction:Settings:geometryTolerance for more '
                      'information')