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/src/v3.rs b/libs/apkverify/src/v3.rs
index fcd966b..e1b728d 100644
--- a/libs/apkverify/src/v3.rs
+++ b/libs/apkverify/src/v3.rs
@@ -110,7 +110,7 @@
 ) -> Result<(Signer, ApkSections<R>)> {
     let mut sections = ApkSections::new(apk)?;
     let mut block = sections.find_signature(APK_SIGNATURE_SCHEME_V3_BLOCK_ID).context(
-        "Fallback to v2 when v3 block not found is not yet implemented. See b/197052981.",
+        "Fallback to v2 when v3 block not found is not yet implemented.", // b/197052981
     )?;
     let mut supported = block
         .read::<Signers>()?
@@ -135,7 +135,7 @@
             .iter()
             .filter(|sig| sig.signature_algorithm_id.map_or(false, |algo| algo.is_supported()))
             .max_by_key(|sig| sig.signature_algorithm_id.unwrap().content_digest_algorithm())
-            .context("No supported signatures found")?)
+            .context("No supported APK signatures found; DSA is not supported")?)
     }
 
     pub(crate) fn find_digest_by_algorithm(
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]