minor: test for navigator.bluetooth to warn on unsupported browsers

[karlp]

It would be "nice" if an unsupported browser (thanks for nothing firefox) would give a warning or something, instead of simply "not doing anything" Currently, connect just gives you this on the console

Uncaught TypeError: navigator.bluetooth is undefined
    connect https://atc1441.github.io/TelinkFlasher.html:87
    onclick https://atc1441.github.io/TelinkFlasher.html:1

[pbasista]

thanks for nothing firefox

I assume that this tool requires the Web Bluetooth functionality in order to work.

Mozilla's positions on adopting features which give the website raw access to the connected devices, like Web USB, Web Bluetooth, Web NFC, and others, are currently broadly negative and they are explained in detail here.

In summary, Mozilla is currently unwilling to implement such features because they find them harmful to the users. One of the main arguments is that the risks which such features pose are non-intuitive to most users and they are also difficult to explain properly in a permissions prompt. Users understand the risks of e.g. installing an unknown application. But they are much less educated about the risks posed by visiting an unknown website and e.g. granting it raw access to its Bluetooth devices.