Improve error message for category dtype in HDF5, format="fixed" store.

A more explicit (and helpful) error message would have helped me discover sooner that format="table" supported categorical data.

Author: chrish42

pandas/io/pytables.py

@@ -2517,7 +2517,7 @@ def write_array(self, key, value, items=None):
         transposed = False
 
         if com.is_categorical_dtype(value):
-            raise NotImplementedError("cannot store a category dtype")
+            raise NotImplementedError('Cannot store a category dtype in a HDF5 dataset that uses format="fixed". Use format="table".')
 
         if not empty_array:
             value = value.T