added chain-matching AlignError

[mgiulini]

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Closes #472 by adding a call to an AlignError. It does not stop caprieval, but rather the capri_ss.tsv file is still produced for all those models that do not show an evident (empty chain) chain-matching error.

[joaomcteixeira]

Great work. Minor comments below! (Sorry i edited my original message)

[joaomcteixeira]

For the pure pythonic way of doing it, I suggest:

if not chain_dic[chain]:

or

if len(chain_dic[chain]) == 0:

[joaomcteixeira]

If you can, please add a minor test covering the new lines:

def test_new_test_function():
    with pytest.raises(ALIGNError):
        load_coords(...)

[joaomcteixeira]

Well done!