Removing ambiguity in PauliRot error message. (#6298)

**Context:**
For the input of `PauliRot` the number of wires given must equal the
length of the given Pauli word. Otherwise a `ValueError` is raised. For
example, `op = qml.PauliRot(0.5, "XY", wires=[0,1,2])` results in
`ValueError: The given Pauli word has length 2, length 3 was expected
for wires [0,1,2]`.

However, this message can be ambiguous. `op = qml.PauliRot(0.5,
"XYZZXYI", wires=[0])` raises the exception `ValueError: The given Pauli
word has length 7, length 1 was expected for wires [0]`. It's unclear if
"length 1" refers to the length of the Pauli word or the length of the
wires.

**Description of the Change:**
This PR changes the error message to remove the ambiguity. Now it will
say `ValueError: The number of wires must be equal to the length of the
Pauli Word. The Pauli word XY has length 2, and 3 wires were given [0,
1, 2].`

doc/releases/changelog-dev.md

@@ -179,6 +179,9 @@
 * Add `work_wires` parameter to `qml.MultiControlledX` docstring signature.
   [(#6271)](https://github.com/PennyLaneAI/pennylane/pull/6271)
 
+* Removed ambiguity in error raised by the `PauliRot` class.
+  [(#6298)](https://github.com/PennyLaneAI/pennylane/pull/6298)
+
 <h3>Bug fixes 🐛</h3>
 
 * `qml.map_wires` can now be applied to a batch of tapes.

pennylane/ops/qubit/parametric_ops_multi_qubit.py

@@ -291,8 +291,9 @@ def __init__(
 
         if not len(pauli_word) == num_wires:
             raise ValueError(
-                f"The given Pauli word has length {len(pauli_word)}, length "
-                f"{num_wires} was expected for wires {wires}"
+                f"The number of wires must be equal to the length of the Pauli word. "
+                f"The Pauli word {pauli_word} has length {len(pauli_word)}, and "
+                f"{num_wires} wires were given {wires}."
             )
 
     def __repr__(self) -> str:

tests/ops/qubit/test_parametric_ops.py

@@ -2995,7 +2995,7 @@ def test_init_incorrect_pauli_word_length_error(self, pauli_word, wires):
 
         with pytest.raises(
             ValueError,
-            match="The given Pauli word has length .*, length .* was expected for wires .*",
+            match=r"The number of wires must be equal to the length of the Pauli word\. The Pauli word .* has length .*, and .* wires were given .*\.",
         ):
             qml.PauliRot(0.3, pauli_word, wires=wires)