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(