Preserve failures during MerlinAU self-update

[maghuro]

Summary

Fix _DownloadScriptFiles_() so that a failed file download cannot be masked by a later successful download.

Problem

retCode is currently overwritten on every successful download.

For example:

1. version.txt download fails -> retCode=1
2. MerlinAU.asp succeeds -> retCode=0
3. MerlinAU.sh succeeds -> retCode=0

The function therefore returns success even though the update was only partially successful.

The same problem exists if the WebUI file fails but the final script download succeeds.

This may cause _SCRIPT_UPDATE_() to report that the MerlinAU files were successfully updated when one of the required downloads actually failed.

Fix

Initialize retCode=0 once at the beginning of _DownloadScriptFiles_().

Successful downloads no longer reset the return code. Any download failure sets retCode=1, and that failure state is preserved for the remainder of the operation.

Result

• all downloads succeed -> returns 0
• version.txt fails -> returns 1
• MerlinAU.asp fails -> returns 1
• MerlinAU.sh fails -> returns 1
• multiple downloads fail -> returns 1

No download, installation, WebUI remount, or firmware-update behavior is otherwise changed.

Scope

This PR intentionally contains only the return-status fix in _DownloadScriptFiles_().

[ExtremeFiretop]

All good, approved and merging.

[ExtremeFiretop]

Nice catch @maghuro !!! Thanks for the help :)
Merged.