shared/install: ignore failures for auxiliary files

If Also= fails, warn, but otherwise ignore the failure.

Fixes #19407.

src/shared/install.h

@@ -33,6 +33,7 @@ enum {
         UNIT_FILE_IS_MASKED,
         UNIT_FILE_IS_DANGLING,
         UNIT_FILE_DESTINATION_NOT_PRESENT,
+        UNIT_FILE_AUXILIARY_FAILED,
         _UNIT_FILE_CHANGE_TYPE_MAX,
         _UNIT_FILE_CHANGE_TYPE_INVALID = -EINVAL,
 };