Raise ConanException if source patch does not exist in export_conandata_patches

[czoido]

Changelog: Fix: Raise ConanException if source patch does not exist in export_conandata_patches.
Docs: Omit

Trying to fix tests that fail in Python 3.13: #17205

I found that now the error thrown by shutil.copy2 has changed in Windows. So if the source patch does not exist now the error is different.

It was like this:

FileNotFoundError: [Errno 2] No such file or directory: '/private/var/folders/hr/t3_1lzj9145g383b4zyj67d40000gn/T/tmpjqj9cq99conans/path with spaces/patches/mypatch.patch'

now it's like this:

FileNotFoundError: [WinError 3] The system cannot find the path specified

So I think it would be better to throw that error ourselves si that we always output a meaningful message:

ConanException: Patch file does not exist: '/private/var/folders/hr/t3_1lzj9145g383b4zyj67d40000gn/T/tmpuewtmlhpconans/path with spaces/patches/mypatch.patch'