metadata API: Tweak exception message on sign failure

I still don't know how we should handle failures in signing
(maybe just let all of the weird exceptions raise instead of wrapping
them) but this makes the wrapping error at least a bit more useful.

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

Author: jku

tuf/api/metadata.py

@@ -384,7 +384,7 @@ def sign(
         try:
             signature = signer.sign(bytes_data)
         except Exception as e:
-            raise UnsignedMetadataError("Problem signing the metadata") from e
+            raise UnsignedMetadataError(f"Failed to sign: {e}") from e
 
         if not append:
             self.signatures.clear()