I get an error message every time I try to install Portal 2 via Steam
Solution 1:
Apparently that message only means the Steam servers are busy, it worked now.
From the Steam website:
Incomplete Installation (10)
Steam servers are busy. Please try again later.
Solution 2:
When I got this, it was because my computer had gone into hibernation during the download. A Portal 2 update came a few seconds later, fixing the problem. Verifying the game cache was unnecessary.