After considering the project location, the web addresses, etc., it is required that the external javascript files (and additional html files) are present along side the manifest.xml when debugging. The taskplane references the javascript files relative to itself in the development location (srcaskpane) i.e.
<script type="text/javascript" src="client.js"></script>
however to debug the add-in, the client.js file must also be in the project root beside the manifest.xml.
Obviously, the other option is to include the external javascript code in your taskpane.js :-)
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