Assert values only when feature enabled
Assert values only when feature enabled

In the cases where `VERIFY` wasn't defined, the variable
`download_files` would never be assigned, causing an error on
access/assert. We shouldn't be comparing values if the feature is
disabled, anyway, so let's move the assertion inside the conditional
block.

stvstnfrd/edx-certificates,Stanford-Online/openedx-certificates,stvstnfrd/edx-certificates,Stanford-Online/openedx-certificates