Implement defensive programming to prevent code breaking

[simondate]

When you install the plugin to the FW it won't work out the box due to its reliance on having Aria label text. There should be some fallback on the following lines to prevent it breaking. Also the current use of the OR operator is redundant since it still breaks if the first one isn't read.

• https://github.com/danielstorey/adapt-stacklist/blob/master/js/adapt-stacklist.js#L107
• https://github.com/danielstorey/adapt-stacklist/blob/master/js/adapt-stacklist.js#L36