[enhancement](schema-change) Record detailed fail reason for schema c…

…hange tasks (apache#39351) (apache#39502)

## Proposed changes

Expose the error msg from BE as the real fail reason recorded for schema
change tasks. To avoid too much memory usage, we just pick one among all
to record.

fe/fe-core/src/main/java/org/apache/doris/alter/SchemaChangeJobV2.java

@@ -522,7 +522,7 @@ protected void runRunningJob() throws AlterCancelException {
                 if (task.getFailedTimes() > 0) {
                     task.setFinished(true);
                     AgentTaskQueue.removeTask(task.getBackendId(), TTaskType.ALTER, task.getSignature());
-                    LOG.warn("schema change task failed: " + task.getErrorMsg());
+                    LOG.warn("schema change task failed: {}", task.getErrorMsg());
                     List<Long> failedBackends = failedTabletBackends.get(task.getTabletId());
                     if (failedBackends == null) {
                         failedBackends = Lists.newArrayList();
@@ -533,8 +533,8 @@ protected void runRunningJob() throws AlterCancelException {
                             .getReplicaAllocation(task.getPartitionId()).getTotalReplicaNum();
                     int failedTaskCount = failedBackends.size();
                     if (expectSucceedTaskNum - failedTaskCount < expectSucceedTaskNum / 2 + 1) {
-                        throw new AlterCancelException("schema change tasks failed on same tablet reach threshold "
-                                + failedTaskCount);
+                        throw new AlterCancelException(
+                                String.format("schema change tasks failed, error reason: %s", task.getErrorMsg()));
                     }
                 }
             }