ENH: emit a useful error message with small threaded regions

include/itkRecursiveLineYvvGaussianImageFilter.hxx

@@ -331,6 +331,14 @@ RecursiveLineYvvGaussianImageFilter< TInputImage, TOutputImage >
 
   const unsigned int ln = region.GetSize()[this->m_Direction];
 
+  if ( ln < 4 )
+    {
+    itkExceptionMacro( "The number of pixels along direction "
+      << this->m_Direction
+      << " in this work-unit image region is less than 4."
+      << " Note: TBBMultiThreader can divide small regions into really small pieces.");
+    }
+
   RealType *inps = nullptr;
   RealType *outs = nullptr;
   RealType *scratch = nullptr;