Improve error message for failed qiskit_to_tk conversion

[CalMacCQ]

Small PR to imporve the error message for failed qiskit_to_tk conversion adressing #150

This snippet now gives the following error message for the unsupported gate

from qiskit import QuantumCircuit
from qiskit.circuit.library import XXPlusYYGate
from pytket.extensions.qiskit import qiskit_to_tk

qc = QuantumCircuit(2)
qc.append(XXPlusYYGate(0.1), [0, 1]) # add unsupported gate

tkc = qiskit_to_tk(qc)

Note: The two reverted commits correspond to an attempted fix of #43. However I realised this was slightly more involved than expected so its probably best adressed in a separate PR.

[CalMacCQ]

Hmm maybe its better to use custom error here rather than NotImplementedError.

[cqc-alec]

Hmm maybe its better to use custom error here rather than NotImplementedError.

NotImplementedError seems fine to me.