Show $ADB_VENDOR_KEYS if authentication fails.

Incorrectly set $ADB_VENDOR_KEYS is the most likely cause of failed
adb connections. Make it easier to debug such problems by including
the value in use in the error message.

Bug: 20165551
Change-Id: I64c1d98ae6d3fb40eea9e1f0ddcfcf4f2d9d7318