improve skip msg when skipping an ECDH test in test_ec (#4355)

diff --git a/tests/hazmat/primitives/test_ec.py b/tests/hazmat/primitives/test_ec.py
index d015e84..4e61b5d 100644
--- a/tests/hazmat/primitives/test_ec.py
+++ b/tests/hazmat/primitives/test_ec.py
@@ -65,8 +65,8 @@
         algorithm, curve
     ):
         pytest.skip(
-            "Exchange algorithm is not supported by this backend {0}".format(
-                backend
+            "Exchange with {0} curve is not supported by {1}".format(
+                curve.name, backend
             )
         )