Include more info in the exception message

Include info about the number of terms actually found.

Author: mhucka

src/openfermion/transforms/opconversions/reverse_jordan_wigner.py

@@ -82,7 +82,10 @@ def reverse_jordan_wigner(qubit_operator, n_qubits=None):
 
                 # Get next non-identity operator acting below 'working_qubit'.
                 if len(working_term.terms) != 1:
-                    raise ValueError('QubitOperator must contain exactly one term')
+                    raise ValueError(
+                        'QubitOperator must contain exactly one term. '
+                        f'Found {len(working_term.terms)} terms: {working_term!r}'
+                    )
                 working_qubit = pauli_operator[0] - 1
                 for working_operator in reversed(list(working_term.terms)[0]):
                     if working_operator[0] <= working_qubit:

src/openfermion/transforms/opconversions/reverse_jordan_wigner_test.py

@@ -166,7 +166,9 @@ def test_reverse_jw_multi_term_error(self):
             'openfermion.transforms.opconversions.reverse_jordan_wigner.' 'QubitOperator.__mul__'
         ) as mock_mul:
             mock_mul.return_value = QubitOperator('X0') + QubitOperator('Y0')
-            with self.assertRaisesRegex(ValueError, 'QubitOperator must contain exactly one term'):
+            with self.assertRaisesRegex(
+                ValueError, 'QubitOperator must contain exactly one term. Found 2 terms'
+            ):
                 reverse_jordan_wigner(QubitOperator('X1'))
 
     def test_jw_convention(self):