RFE: metadata/metarefresh error handling improvements

Currently the eduGAIN feed contains an SP with empty <mdui:Logo>. When running Metarefresh, this makes mdui\Logo throw an exception, saying Missing url value for Logo. Now that's okay, although it doesn't tell me anything about which entity it is.

But 1) the exception propagates all the way upwards, cancelling the import of the entire feed – meaning that just about any SP anywhere can accidentally break my whole eduGAIN access. (Somehow even eduGAIN didn't catch this error.) Maybe it would be possible to skip just that one entity – or even load it without any logos, as they're probably not crucial to the operation.

And 2) the exception is eventually swallowed and thrown away in MetaLoader, which instead shows a very generic "XML parser error when parsing https://fedi.litnet.lt/metadata/edugain.xml - attempting to re-use cached metadata". I spent quite some time trying to figure out that one, as the file is perfectly valid XML. I think it would be good to show the actual exception that was caught.

Issue で説明されている Metarefresh のインポートパスと MetaLoader のエラーハンドリングのエントリーポイントから開始してください。無効な mdui:Logo が 1 つのエンティティにどのような影響を与えるか、また捕捉された例外がどのように汎用的なキャッシュ済みメタデータのメッセージになるかを追跡してください。1 つの不正なエンティティによってフィードのインポートがキャンセルされず、基となる例外が報告されたエラーに保持されれば完了です。