To reproduce:

1. Open the project, attached by the user (404Test.zip)
2. Open the "Test" scene
3. Enter Play mode
4. Observe the console
5. (Optional) Observe the Test.cs script

Expected: loading a non-existing file with the same name as previously loaded file should make isNetworkError (or isHttpError) true
Actual: if a file "blanket.assetbundle" is loaded successfully, then loading the file "blanket.unity3d" shows no errors about it not existing, and you still can try downloading it with DownloadHandlerAssetBundle

Reproduced in 2017.1.2p2, 2017.2.0p1, 2017.3.0b8, 2018.1.0a3
Cound not test for regression, in 5.6 UnityWebRequest doesn't have isNetworkError or isHttpError fields.