Should throw error if external_choices list is not found

[joeflack4]

Notes
This is a duplicate of: XLSForm/pyxform#220
This is a duplicate of: jkpr/QTools2#17

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Description

When there is a question of type select_one external_choices LIST or select_multiple external_choices LIST where "LIST" is not found within the "external_choices" worksheet of an XlsForm, an error should be thrown and the conversion halted.

Expected behavior

When issue occurs, an error should be thrown and the conversion halted.

Actual behavior

When issue occurs, the form converts normally. Then, when the user is interacting with the actual form and gets to a question with a missing list, the choice options do not appear.