Fix QuantumError sometimes casting probabilities to complex

[ghost]

Summary

This PR is currently blocked by bugs in Aer. This PR will hopefully fix some of those bugs.

Details and comments

Currently, Noise Models are being received on the C++ side with probability attributes having (what appears to be) complex types - we have yet to identify why but the existing changes will at least raise a reasonable exception when it does happen.

Bug was caused by a missing cast from complex type to float type in a QuantumError factory. Thanks @chriseclectic

[mtreinish]

Thanks for starting this, the other thing which would probably be good here is to add a test case that runs something like:

from qiskit import execute
from qiskit import QuantumCircuit
from qiskit.test import mock
from qiskit.providers import aer
from qiskit.providers.aer.noise import NoiseModel

circ = QuantumCircuit(2)
circ.x(0)
circ.x(1)
circ.measure_all()

backend = mock.FakeSingapore()
noise_model = NoiseModel.from_backend(backend)
qobj = assemble(transpile(circ, backend), backend)
sim = aer.QasmSimulator()
sim.run(qobj, noise_model=noise_model).result()

to ensure we don't regress here moving forward.

[mtreinish]

LGTM, just one stray comment in there then I think we're good to merge this.

[mtreinish]

LGTM, thanks for updating