backup: stop main thread if reader thread dies unexpectedly

_thread.interrupt_main will trigger KeyboardInterrupt in the main thread.

backup.py

@@ -15,6 +15,7 @@ import select
 import pathlib
 import threading
 import subprocess
+import _thread # for interrupt_main
 
 import typing
 
@@ -359,7 +360,14 @@ def main(argv: typing.List[str]):
                 sys.stdout.flush()
                 captured_output.append(line)
             fh.close()
-        reader = threading.Thread(target=reader_thread, args=(borg.stdout,))
+        def _reader_thread(fh):
+            try:
+                return reader_thread(fh)
+            except BrokenPipeError:
+                pass
+            except Exception:
+                _thread.interrupt_main()
+        reader = threading.Thread(target=_reader_thread, args=(borg.stdout,))
         reader.daemon = True
         reader.start()