improve error reporting when compiled without libusb
improve error reporting when compiled without libusb

ipod-read-sysinfo-extended error message was wrongly referring to a
missing libimobiledevice when trying to read SysInfoExtended from USB
with no libusb support.

neuschaefer/libgpod,hyperair/libgpod,hyperair/libgpod,hyperair/libgpod,neuschaefer/libgpod,neuschaefer/libgpod,neuschaefer/libgpod,hyperair/libgpod,hyperair/libgpod,neuschaefer/libgpod