Add more context for avb::Header::set_algo_for_key() calls

Previously, it was not always obvious which key was problematic.

Issue: #366

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

avbroot/src/cli/avb.rs

@@ -398,7 +398,9 @@ fn sign_or_clear(info: &mut AvbInfo, orig_header: &Header, key_group: &KeyGroup)
                 RsaSigningKey::Internal(private_key)
             };
 
-            info.header.set_algo_for_key(&signing_key)?;
+            info.header
+                .set_algo_for_key(&signing_key)
+                .context("Failed to set signature algorithm")?;
             info.header
                 .sign(&signing_key)
                 .context("Failed to sign new AVB header")?;

avbroot/src/cli/ota.rs

@@ -593,7 +593,9 @@ fn update_vbmeta_headers(
         // have no dependencies and are only being processed to ensure that the
         // flags are set to a sane value.
         if parent_header != &orig_parent_header {
-            parent_header.set_algo_for_key(key)?;
+            parent_header
+                .set_algo_for_key(key)
+                .with_context(|| format!("Failed to set signature algorithm: {name}"))?;
             parent_header
                 .sign(key)
                 .with_context(|| format!("Failed to sign vbmeta header for image: {name}"))?;