Q: MacKeeper - do I have a virus or not?

Good that you did not install MacKeeper.

You likely just got a popup since they wanted you to install something.

To check for unwanted stuff:

First try

How to install (Really remove) adware

https://discussions.apple.com/docs/DOC-7471

Stop pop-up ads and adware in Safari

https://support.apple.com/en-us/HT203987

Adware Removal Guide : Identification

http://www.thesafemac.com/arg-identification/

Next if necessary:

Malwarebytes Anti-Malware for Mac

https://www.malwarebytes.com/antimalware/mac/

https://www.malwarebytes.com/mac-download/

Tapsnake is not a virus, but what is known as adware - a program designed to make you think you have a virus and install a completely unnecessary, and indeed harmful, program to deal with it. This probably isn't even the real Mackeeper (which itself is bad enough). You should reset Safari to get rid of all this, and do not run MacKeeper, which will probably install worse malware. Please see this page for more details:

http://macsecurity.net/view/123/