Installing tensorflow-gpu on a Laptop with two graphic cards
At what phase are you getting the "Download wasn't..." message? did
  you try manually downloading the wheel file and installing it directly
  and locally? – Ido_f

Downloading the local CUDA installation solved my issues.
A lot of people have issues installing tensorflow-gpu on their computers and I have read a lot of them and tried out a lot of them as well. So I'm not coming for an easy answer without searching the web beforehand. 
I'm running W10 with an NVIDIA Quadro P600 which can supposedly run CUDA. 
The thing is whenever I'm trying to install CUDA (10.0 as suggested from tensorflow) the installation breaks with no clear indication of the error ("The download wasn't successfully completed. Try again") 
I have the feeling it breaks because my Laptop has two GPU's. The mentioned NVIDIA and the onboard Intel UHD Graphics 630 card.
Does anyone have a clue? Please share your workflow if you have installed tensorflow-gpu on your laptop!