Upgrade failed to download from Mavericks to Yosemite
That's good that you're approaching things cautiously! However, you do not need to worry about this; Apple has built in multiple levels of protection to prevent you from messing up your computer. 

There are two larger protections:


The App store automatically makes sure that the file you downloaded is what you were supposed to download and fixes anything it sees wrong.
When installing, the Yosemite installer double checks that the installation files are all in order and stops the installation if not.


Therefore, you should be okay to just go for it and upgrade.