The version of T3Main.exe that it attempts to download is the newest version, which comes with the Thief 3 level editor. It's the only one which works properly with the extra levels (fanmissions) made by the Thief community. (I should probably add the explanation to that dialog.)
The download should work - wait a while and try again, or get the
Editor Pack (~300 MB) and copy the file called T3MainReleaseVersion.exe into the System folder in whatever folder Steam installed the game in. Rename it T3Main.exe (replacing the existing one), then try the Sneaky Upgrade installer again.