Return FileExistsException and FileException as data on FileUploadResult

Problem/Motivation

FileUploadHandler::handleFileUpload() throws FileExistsException when the destination file exists and FileException for an invalid filename. Callers must catch these separately from checking violations on the result object, mixing two error-reporting styles for failures caused by user input.

Steps to reproduce

Proposed resolution

Add getErrors() to FileUploadResult, separate from getViolations(). handleFileUpload() catches FileExistsException and FileException internally and adds them to the result's errors instead of throwing. InvalidStreamWrapperException, FileWriteException, and LockAcquiringException keep throwing, since they reflect misconfiguration or environment failures rather than user input.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet