|Seen this message before? Can't play your games? Well, read on...|
I've noticed that this particular error is getting more and more common on Steam. Basically, what happens, is that Steam tries to update a game in some fashion and ends up getting a glitch and is unable to proceed. The error has been around for quite some time now and it's trivially simple to fix if you know what to do and where to look... in which case the fact that Valve have no Steam Help page dedicated to the issue (at least not one that doesn't assume your PC or HDD is in the process of dying).
Even worse, it would be pretty easy to code in the fix into the Steam client as a 'when this happens, do this' bit of functionality - all the indicators are there already so the information would just need to be flagged and acted upon.
I looked for quite some time, saw many incorrect solutions and many unhelpful 'suggestions' from hopefully well-meaning users and finally found something that worked (although I don't remember exactly which youtube video it was. I didn't find the work-around written anywhere though - so that's where this PSA comes in...
Now, I'm not going to say that this solution will definitively work for all causes of this error. If your HDD is dying or there is a problem with your system that is causing problems with writing to the HDD then you will need to address that in your own way. Otherwise, this is a good guide to start with before doing crazy things like reinstalling the game, Steam and your OS!