No variables read in

[znicholls]

Describe the bug

If you set only="junk" when running MAGICC, the error is too cryptic.

Failing Test

with pytest.raises(ValueError, match="No variables found for only = 'junk', returning an empty dataframe"):
    with MAGICC6() as magicc:
        magicc.run(ony="junk")

Expected behavior

See test above

Screenshots

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System (please complete the following information):

• OS: Linux
• Python and Pymagicc version: Python 3.7

Additional context

Add any other context about the problem here.