When a topology command (e.g., rebuild) fails on a target node, the
exception message was being swallowed at multiple levels:
1. raft_topology_cmd_handler caught exceptions and returned a bare
fail status with no error details.
2. exec_direct_command_helper saw the fail status and threw a generic
"failed status returned from {id}" message.
3. The rebuilding handler caught that and stored a hardcoded
"streaming failed" message.
This meant users only saw "rebuild failed: streaming failed" instead
of the actionable error from the safety check (e.g., "it is unsafe
to use source_dc=dc2 to rebuild keyspace=...").
Fix by:
- Adding an error_message field to raft_topology_cmd_result (with
[[version 2026.2]] for wire compatibility).
- Populating error_message with the exception text in the handler's
catch blocks.
- Including error_message in the exception thrown by
exec_direct_command_helper.
- Passing the actual error through to rtbuilder.done() instead of
the hardcoded "streaming failed".
A follow-up test is in https://github.com/scylladb/scylladb/pull/29363
Fixes: SCYLLADB-1404
Closesscylladb/scylladb#29362
3cba27d25f