overall better handling and usage of exceptions

bundles/apt/items.py

@@ -1,3 +1,5 @@
+from bundlewrap.exceptions import BundleError
+
 supported_os = {
     'debian': {
         10: 'buster',
@@ -9,7 +11,10 @@ supported_os = {
     },
 }
 
-assert supported_os[node.os][node.os_version[0]], '{}: OS {} {} is not supported by bundle:apt'.format(node.name, node.os, node.os_version)
+try:
+    supported_os[node.os][node.os_version[0]]
+except (KeyError, IndexError):
+    raise BundleError(f'{node.name}: OS {node.os} {node.os_version} is not supported by bundle:apt')
 
 
 actions = {