Metadata API: Add messages to serialization errors

We can't really add any details but this at least means
printing the error works.

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

Author: Jussi Kukkonen

tuf/api/serialization/json.py

@@ -36,7 +36,7 @@ def deserialize(self, raw_data: bytes) -> Metadata:
             metadata_obj = Metadata.from_dict(json_dict)
 
         except Exception as e:
-            raise DeserializationError from e
+            raise DeserializationError("Failed to deserialize JSON") from e
 
         return metadata_obj
 
@@ -66,7 +66,7 @@ def serialize(self, metadata_obj: Metadata) -> bytes:
             ).encode("utf-8")
 
         except Exception as e:
-            raise SerializationError from e
+            raise SerializationError("Failed to serialize JSON") from e
 
         return json_bytes