Ignore missing initializers

Previously, the startup_scripts would fail if an initializer file was
not present. Now they just ignore missing files.

startup_scripts/050_manufacturers.py

@@ -1,7 +1,13 @@
 from dcim.models import Manufacturer
 from ruamel.yaml import YAML
+from pathlib import Path
+import sys
 
-with open('/opt/netbox/initializers/manufacturers.yml', 'r') as stream:
+file = Path('/opt/netbox/initializers/manufacturers.yml')
+if not file.is_file():
+  sys.exit()
+
+with file.open('r') as stream:
   yaml = YAML(typ='safe')
   manufacturers = yaml.load(stream)