Fixes Issue #3505: Raise AttributeError on setting mask on unmasked column - #3522

@sahmed95 - looks like a good start, but you also need an explicit test of the exception being raised as expected. This should go in test_column.py.

Updated the commit: Added Test Case and followed PEP8

@sahmed95 - there are widespread failures from this. You should work on this and make sure tests are passing locally before pushing changes up to github, for instance:

python setup.py test --package=table --args='--pdb'`

There seems to be a lot of places where mask is set on unmasked column by a functions in different modules.

___ TestFormatWithMaskedElements.test_column_format_func_with_special_masked ___
self = <astropy.table.tests.test_pprint.TestFormatWithMaskedElements instance at 0x1147d5a70>
    def test_column_format_func_with_special_masked(self):
        # run most of functions twice
        # 1) astropy.table.pprint._format_funcs gets populated
        # 2) astropy.table.pprint._format_funcs gets used

        t = Table([[1., 2., 3.], [3, 4, 5]], names=('a', 'b'), masked=True)
>       t['a'].mask = [True, False, True]
astropy/table/tests/test_pprint.py:445: 

The official numpy docs do not recommend the use of ".mask" to set mask value and instead use ma.mask :

From http://docs.scipy.org/doc/numpy/reference/maskedarray.generic.html :

A second possibility is to modify the mask directly, but this usage is discouraged.

Raising AttributeError while trying to set mask on an unmasked column might not be the best way then.
Is the issue more about warning the user that he/she is trying to set mask on a column which is not a MaskedColumn object ?

If that is the case then should a simple warning message be displayed saying that the user should probably use MaskedColumn if intending to set mask rather than raising AttributeError.
Else, a lot of changes have to be made in multiple places to accommodate this change.

Incorrect implementation.