Reject/throw as appropriate in ES6 exported methods if window.fdc3 is not available

[mattjamieson]

Improve error handling in exported methods by:

• rejecting with the message 'FDC3 DesktopAgent not available at 'window.fdc3'. if the method returns a Promise (e.g. open), and
• throwing with the same message if the method is synchronous (e.g. broadcast).

At present, developers would see the significantly less helpful:

Uncaught TypeError: Cannot read property 'broadcast' of undefined

This can be improved further in future by leveraging the ready event pattern proposed in #269.

[rikoe]

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