Add an error code for generic hardware error.

This is necessary because currently the wifi code just returns
whatever hardware-specific integer it gets back from the HAL,
which is bad because that will be interpreted by the caller as
one of the error codes defined in this class.

In parallel we'll also modify the wifi code to return this new
error code if the hardware returns an error.

Bug: 21405946
Change-Id: Ic9fa1193ced69a4e7ff543e397221c89b10a5a13