only set letterhead if it exists on the filesystem
only set letterhead if it exists on the filesystem

cecep-edu/edx-certificates,eestay/edx-certificates,appsembler/edx-certificates,Stanford-Online/openedx-certificates,victoria-valdes/edx-certificates,stvstnfrd/edx-certificates,olexiim/edx-certificates,victoria-valdes/edx-certificates,olexiim/edx-certificates,EDUlib/edx-certificates,stvstnfrd/edx-certificates,EDUlib/edx-certificates,cecep-edu/edx-certificates,Stanford-Online/openedx-certificates,edx/edx-certificates,marcore/edx-certificates,edx/edx-certificates,appsembler/edx-certificates,ESOedX/edx-certificates,eestay/edx-certificates,ESOedX/edx-certificates,marcore/edx-certificates