ducktape: Capture parse failures explicitly

Since the compute storage script runs over ssh and the output is
expected to be JSON, the errors need to be presented as a special JSON
structure.

tests/rptest/remote_scripts/compute_storage.py

@@ -119,6 +119,7 @@ def compute_size_for_file(file: Path, calc_md5: bool):
 def compute_size(data_dir: Path, sizes: bool, calculate_md5: bool,
                  print_flat: bool):
     output = {}
+    segment_parse_error = {}
     for ns in safe_listdir(data_dir):
         if not safe_isdir(ns):
             continue
@@ -144,6 +145,14 @@ def compute_size(data_dir: Path, sizes: bool, calculate_md5: bool,
                             # It's valid to have a segment deleted
                             # at anytime
                             continue
+                        except AssertionError as e:
+                            if print_flat:
+                                raise e
+                            return {
+                                "parse_failed": True,
+                                "segment": segment,
+                                "error": str(e)
+                            }
                     part_output[segment.name] = seg_output
                 topic_output[partition.name] = part_output
             ns_output[topic.name] = topic_output