Migrate NFS KIO worker to KIO::WorkerBase

Some necessary changes included

• Making every function that called setError return WorkerResult
• Making getFileHandle return an expected/result-like result to ensure any errors getting the file handle are passed along to the caller
• Update virtual method implementations to return WorkerResult

Due to the vast differences in error reporting between KF5's SlaveBase and KF6's WorkerBase, some of the error logic might have been altered