Clarify error message

Make it clear we're talking about APK signatures and what isn't
supported.

Don't include a bug number in a message that non-Googlers may see.

Bug: 243512240
Test: atest libapkverify.test
Test: atest libapkverify.integration_test
Change-Id: I80d3807e845c27f57b780f0f297424e62f36158d
diff --git a/libs/apkverify/tests/apkverify_test.rs b/libs/apkverify/tests/apkverify_test.rs
index 047538c..baf7c42 100644
--- a/libs/apkverify/tests/apkverify_test.rs
+++ b/libs/apkverify/tests/apkverify_test.rs
@@ -44,7 +44,7 @@
     for key_name in KEY_NAMES_DSA.iter() {
         let res = verify(format!("tests/data/v3-only-with-dsa-sha256-{}.apk", key_name));
         assert!(res.is_err(), "DSA algorithm is not supported for verification. See b/197052981.");
-        assert_contains(&res.unwrap_err().to_string(), "No supported signatures found");
+        assert_contains(&res.unwrap_err().to_string(), "No supported APK signatures found");
     }
 }
 
@@ -151,7 +151,7 @@
 fn test_verify_v3_no_supported_sig_algs() {
     let res = verify("tests/data/v3-only-no-supported-sig-algs.apk");
     assert!(res.is_err());
-    assert_contains(&res.unwrap_err().to_string(), "No supported signatures found");
+    assert_contains(&res.unwrap_err().to_string(), "No supported APK signatures found");
 }
 
 #[test]