I have the same problem.
I updated a secondary iMac today to 10.10.3 using the app store Update pane (an iMac on which my main iPhoto library does NOT live, just to be safe...)
after updating, I see the new Photos app, but cannot launch the old iPhoto app (v9.6) - I see the same error message as the OP when I try to launch it.
also, I notice that the old iPhoto application icon now has a white circle with diagonal slash - the kind of icon we saw back when PowerPC apps stopped being supported by the OS
so, contrary to previous articles that we'd be able to continue using the old iPhoto app after the new Photos app came out, this doesn't seem to hold true if updated to 10.10.3
p.s. while iPhoto no longer works, I have Aperture 3.6 and that still launches ok under 10.10.3
Based on advice I got in the Apple support forums, delete iPhoto and then redownload it from the Purchases tab in the App Store. That properly downloaded iPhoto 9.6.1 for me. My guess is that Aperture would behave the same way.