backup: track errors/warnings from borg; add prefix to them

This also ignores the "file changed while we backed it up" error, because
that isn't important enough to warrant sending an email.

backup.py

@@ -304,6 +304,9 @@ def main(argv: list[str]):
         if borg.stdin is None:
             raise Exception("no pipe")
 
+        borg_saw_warnings = 0
+        borg_saw_errors = 0
+
         # Use a thread to capture output
         def reader_thread(fh):
             last_progress = 0
@@ -313,7 +316,20 @@ def main(argv: list[str]):
                     if ((data['type'] == 'log_message' or
                          data['type'] == 'progress_message')
                         and 'message' in data):
-                        line = (data['message'] + '\n').encode()
+
+                        # Count warnings and errors, but ignore some.
+                        changed_msg = "file changed while we backed it up"
+                        if data['levelname'] == 'WARNING':
+                            prefix = "warning: "
+                            if changed_msg not in data['message']:
+                                borg_saw_warnings += 1
+                        elif data['levelname'] not in ('DEBUG', 'INFO'):
+                            prefix = "error: "
+                            borg_saw_errors += 1
+                        else:
+                            prefix = ""
+
+                        line = (prefix + data['message'] + '\n').encode()
                     elif data['type'] == 'archive_progress':
                         now = time.time()
                         if now - last_progress > 10:
@@ -359,8 +375,12 @@ def main(argv: list[str]):
         ret = borg.returncode
         if ret < 0:
             backup.log('E', f"borg exited with signal {-ret}")
-        elif ret != 0:
-            backup.log('E', f"borg exited with return code {ret}")
+        elif ret == 2 or borg_saw_errors:
+            backup.log('E', f"borg exited with errors (ret={ret})")
+        elif ret == 1 or borg_saw_warnings:
+            backup.log('W', f"borg exited with warnings (ret={ret})")
+        else:
+            backup.log('E', f"borg exited with unknown error code {ret}")
 
     # See if we had any errors
     warnings = sum(1 for (letter, msg) in backup.logs if letter == 'W')