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: Christian Hudon <[email protected]>

Closes #13093 from chrish42/patch-1 and squashes the following commits:

9a6ef08 [Christian Hudon] Wrap line to be less than 80 characters.
8f436dc [Christian Hudon] Improve error message for category dtype in HDF5, format="fixed" store.

Author: chrish42

pandas/io/pytables.py

@@ -2517,7 +2517,9 @@ 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