fastboot: gracefully handle failure to open a USB device on OS X.

High Sierra restricts opening some USB devices (e.g. the touchbar)
to processes that have specific entitlements. Ignore devices that we
can't open.

Bug: http://b/64292422
Test: manual
Change-Id: I6074b53a365b8d936610bafea60244f8bba1a33f
1 file changed