the "failed to charge insufficient power" is because the N900 is one of the few devices really following the USB - Specifications. It NEEDS the System to allow high power consumption (wich is why it doesnt work with most dummy-chargers btw)
If the System isnt booted or fails to respond fast enough to the N900s request for high power (or something else in the communication doesnt work), it wont charge.
But thats something different i think. Anyway, ive charged a completly empty battery with the USB Charger several times without any problem.