backup: print exceptions from reader thread

_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
 
@@ -317,10 +318,7 @@ def main(argv: typing.List[str]):
             for line in fh:
                 try:
                     data = json.loads(line)
-                    if ((data['type'] == 'log_message' or
+                    if data['type'] == 'log_message':
-                         data['type'] == 'progress_message')
-                        and 'message' in data):
-
                         # Count warnings and errors, but ignore some.
                         changed_msg = "file changed while we backed it up"
                         if data['levelname'] == 'WARNING':
@@ -334,6 +332,11 @@ def main(argv: typing.List[str]):
                             prefix = ""
 
                         line = (prefix + data['message'] + '\n').encode()
+
+                    elif (data['type'] == 'progress_message'
+                          and 'message' in data):
+                        line = (data['message'] + '\n').encode()
+
                     elif data['type'] == 'archive_progress':
                         now = time.time()
                         if now - last_progress > 10:
@@ -353,13 +356,20 @@ def main(argv: typing.List[str]):
                         # ignore unknown progress line
                         continue
                 except Exception as e:
-                    # on error, print raw line
+                    # on error, print raw line with exception
-                    pass
+                    line = f"[exception: {str(e)} ]".encode() + line
                 sys.stdout.buffer.write(line)
                 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()