STYLE: Allow a whitespace after variable in warning macro message

Allow a whitespace between the variable and the remaining sentence part
in exception macro message.

Modules/Filtering/DiffusionTensorImage/include/itkDiffusionTensor3DReconstructionImageFilter.h

@@ -249,7 +249,7 @@ class ITK_TEMPLATE_EXPORT DiffusionTensor3DReconstructionImageFilter
   {
     if (idx >= m_NumberOfGradientDirections)
     {
-      itkExceptionMacro(<< "Gradient direction " << idx << "does not exist");
+      itkExceptionMacro(<< "Gradient direction " << idx << " does not exist");
     }
     return m_GradientDirectionContainer->ElementAt(idx);
   }