Provide warning/info if poked signal doesn't exist

I'd suggest providing some feedback to the user if the signal they're trying to poke doesn't actually exist. The simple example below illustrates the issue and how it could potentially mask a real bug.

import magma as m
import fault

class DefinitelyAnInverter(m.Circuit):
    IO = ["a", m.In(m.Bit),
          "b", m.Out(m.Bit)]

    @classmethod
    def definition(io):
        # this line is definitely right
        io.b <= io.a

tester = fault.Tester(DefinitelyAnInverter)

########## issue here
tester.circuit.aa = 1
#####################

tester.eval()
tester.circuit.b.expect(0)

tester.compile_and_run("verilator", flags=["-Wno-fatal"], directory="build")