Ffmpeg versioning issues (#68)

* FFMPEG version cleanup

* Make sure attempts don't go on forever

* Use latest version when version not defined. Add latest to UI

src/engines/core/base_engine.py

@@ -13,7 +13,7 @@ class BaseRenderEngine(object):
 
     def __init__(self, custom_path=None):
         self.custom_renderer_path = custom_path
-        if not self.renderer_path():
+        if not self.renderer_path() or not os.path.exists(self.renderer_path()):
             raise FileNotFoundError(f"Cannot find path to renderer for {self.name()} instance")
 
     def renderer_path(self):