[web-midi-api] requestMIDIAccess will raise "InvalidSecurityError" (#78)

Underlying systems may raise an error on initialization.
JavaScript should be able to catch the error.

(To close issue #77)
You can merge this Pull Request by running:

  git pull https://github.com/toyoshim/web-midi-api request_invalid_state_error

Or you can view, comment on it, or merge it online at:

  https://github.com/WebAudio/web-midi-api/pull/78

-- Commit Summary --

  * requestMIDIAccess will raise "InvalidSecurityError"

-- File Changes --

    M index.html (3)

-- Patch Links --

https://github.com/WebAudio/web-midi-api/pull/78.patch
https://github.com/WebAudio/web-midi-api/pull/78.diff