Metadata API: Log details of verify error

We don't want to error out from the whole verify_delegate() process if
e.g. a single key fails to load but we do want to provide details for
debugging in the unexpected failure cases.

This means "example_client -vv  download file1.txt" fails like this:

    Found trusted root in /home/jku/.local/share/python-tuf-client-example
    INFO:tuf.api.metadata:Key
4e777de0d275f9d28588dd9a1606cc748e548f9e22b6795b7cb3f63f98035fcb failed
to verify sig: Failed to load PEM key bogus-key-content-here
    INFO:tuf.api.metadata:Key 4e777de0d275f9d28588dd9a1606cc748e548f9e22b6795b7cb3f63f98035fcb failed to verify root
    Failed to download target x: root was signed by 0/1 keys

Fixes #1875

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tuf/api/metadata.py

@@ -704,6 +704,8 @@ def verify_signature(
             sslib_exceptions.UnsupportedAlgorithmError,
             SerializationError,
         ) as e:
+            # Log unexpected failure, but continue as if there was no signature
+            logger.info("Key %s failed to verify sig: %s", self.keyid, str(e))
             raise exceptions.UnsignedMetadataError(
                 f"Failed to verify {self.keyid} signature"
             ) from e