CANPacker: some exceptions are silent

[sshane]

For example, since we don't propagate exceptions from this function, if you pass certain inputs to make_can_msg (like a list instead of dict), you'll get an exception that Python never knows about. But do we never want to throw an exception? Related: #500

opendbc/can/packer_pyx.pyx

Lines 29 to 39 in 3ef35ed

	cdef vector[uint8_t] pack(self, addr, values):
	cdef vector[SignalPackValue] values_thing
	values_thing.reserve(len(values))
	cdef SignalPackValue spv
	for name, value in values.iteritems():
	spv.name = name.encode("utf8")
	spv.value = value
	values_thing.push_back(spv)
	return self.packer.pack(addr, values_thing)

[sshane]

@adeebshihadeh I think this is worth raising an exception for, rather than adding to the valid flag for wrong signals we want to add. What do you think?