Improve the text of an error message in weighted_pauli_operator.py (Qiskit/qiskit-aqua#892)
Improve the text of an error message in weighted_pauli_operator.py (Qiskit/qiskit-aqua#892)

* Improve the text of the error message

Co-authored-by: Manoel Marques <726dfbbf2d82742fc601e72b7538ea72c4fcf7b0@us.ibm.com>
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py