Note:
This issue is only reproducible when trying to download a large asset and there is not enough free disk space available.

Steps to reproduce:
1. Create a new project.
2. Try downloading this asset, for example:
https://www.assetstore.unity3d.com/en/#!/content/39948
3. Notice that after some time the process of downloading fails and the "Console" window outputs:
"success && actual == (UInt64)size error"

Expected result:
Before the process of downloading begins, Unity should check if there is enough space on disk available and do not start downloading the asset until there is enough free space available (it is more like a future request). Also, this error message does not provide the user with useful information (it should be changed into something more comprehensible).

Workaround:
Have enough free disk space.

Reproduced with:
5.2.4f1, 5.3.5f1, 5.4.0b18.

Cannot reproduce with:
5.1.0f3 (because the asset is not supported).

Picture attached.