requestMIDIAccess will raise "InvalidSecurityError"

[toyoshim]

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

(To close issue #77)

[marcoscaceres]

If requestMIDIAccess() is used with a promise, then the promises' error resolver(?) should be invoked with the error instead (i.e., an exception should not be thrown).

[toyoshim]

Oh, yes. My description was bad. It contained confusing keywords, "raise" and "catch"...
Actually, Chrome invokes promises' error resolver with "InvalidSecurityError".

[cwilso]

I'm not going to hack in the pull request (it won't auto-merge), but I put this into the latest version I'm about to check in.