IpSecManagerTest: clean up assertion for better error reporting

See the following flaky failure:
https://android-build.corp.google.com/test_investigate/?invocationId=I59900010160325881&testResultId=TR98428662107002537

Bug: 270193575
Test: TH
Change-Id: I90dfbbec49d86fbf61d378f0ae98ddde5f27bf1f
diff --git a/tests/cts/net/src/android/net/cts/IpSecManagerTest.java b/tests/cts/net/src/android/net/cts/IpSecManagerTest.java
index f935cef..cc0a5df 100644
--- a/tests/cts/net/src/android/net/cts/IpSecManagerTest.java
+++ b/tests/cts/net/src/android/net/cts/IpSecManagerTest.java
@@ -457,9 +457,8 @@
             long newUidRxPackets = TrafficStats.getUidRxPackets(Os.getuid());
 
             assertEquals(expectedTxByteDelta, newUidTxBytes - uidTxBytes);
-            assertTrue(
-                    newUidRxBytes - uidRxBytes >= minRxByteDelta
-                            && newUidRxBytes - uidRxBytes <= maxRxByteDelta);
+            assertTrue("Not enough bytes", newUidRxBytes - uidRxBytes >= minRxByteDelta);
+            assertTrue("Too many bytes", newUidRxBytes - uidRxBytes <= maxRxByteDelta);
             assertEquals(expectedTxPacketDelta, newUidTxPackets - uidTxPackets);
             assertEquals(expectedRxPacketDelta, newUidRxPackets - uidRxPackets);
         }