Error if any of the packages in USE-PACKAGE do not exist.
Error if any of the packages in USE-PACKAGE do not exist.

Previously this would push NIL on the package's use list, leading to
failures later on.

rvedam/Mezzano,varjagg/Mezzano,varjagg/Mezzano,rvedam/Mezzano,froggey/Mezzano,AugustusHuang/Mezzano,AugustusHuang/Mezzano,froggey/Mezzano