add NotFoundWarning

docs/internals/frontends.rst

@@ -721,6 +721,8 @@ Warnings
         {}: {}
     IOWarning rc: 106
         {}: {}
+    NotFoundWarning rc: 107
+        {}: {}
 
 Operations
     - cache.begin_transaction

src/borg/helpers/errors.py

@@ -128,6 +128,8 @@ class BackupOSWarning(BorgWarning):
         assert isinstance(exc, BackupOSError)
         if exc.errno in (errno.EPERM, errno.EACCES):
             return PermissionWarning.exit_mcode
+        elif exc.errno in (errno.ENOENT,):
+            return NotFoundWarning.exit_mcode
         elif exc.errno in (errno.EIO,):
             return IOWarning.exit_mcode
         else:
@@ -146,6 +148,12 @@ class IOWarning(BorgWarning):
     exit_mcode = 106
 
 
+class NotFoundWarning(BorgWarning):
+    """{}: {}"""
+
+    exit_mcode = 107
+
+
 class BackupError(Exception):
     """
     Exception raised for non-OSError-based exceptions while accessing backup files.