hdl.ast: clarify exception message for out of bounds indexing.

Fixes #488.

nmigen/hdl/ast.py

@@ -254,7 +254,7 @@ class Value(metaclass=ABCMeta):
         n = len(self)
         if isinstance(key, int):
             if key not in range(-n, n):
-                raise IndexError("Cannot index {} bits into {}-bit value".format(key, n))
+                raise IndexError(f"Index {key} is out of bounds for a {n}-bit value")
             if key < 0:
                 key += n
             return Slice(self, key, key + 1)

tests/test_hdl_ast.py

@@ -187,7 +187,7 @@ class ValueTestCase(FHDLTestCase):
         self.assertEqual(s2.start, 3)
         self.assertEqual(s2.stop, 4)
         with self.assertRaisesRegex(IndexError,
-                r"^Cannot index 5 bits into 4-bit value$"):
+                r"^Index 5 is out of bounds for a 4-bit value$"):
             Const(10)[5]
 
     def test_getitem_slice(self):