Raise TypeError for invalid Dataset instance

[koteshyelamati]

Problem

stack() and stack_cols() silently return None when passed a non-Dataset argument:

ds = tablib.Dataset()
result = ds.stack("not a dataset")
print(result)  # None — caller has no idea the operation failed

This makes bugs very hard to diagnose: callers receive None without any indication that the argument was invalid.

Fix

Raise TypeError with a clear message instead of silently returning None.

ds = tablib.Dataset()
ds.stack("not a dataset")
# TypeError: 'other' must be a Dataset instance