Define errors for nonreducable and to small learnblocks

cml/shared/errors.py

@@ -10,3 +10,11 @@ class NoModelReconstructedError(BaseError):
 
 class DeconstructionFailed(BaseError):
     pass
+
+
+class NotEnoughFeaturesWarning(BaseError):
+    pass
+
+
+class NotReducableLearnblockWarning(BaseError):
+    pass