need to print detailed error when backup fails

[shanlai]

Your Environments (required)
[vesoft@sandy data]$ ./br version
Nebula Backup And Restore Utility Tool,V-0.6.1
GitSha: 3c8c1cd-dirty
GitRef: update_version

How To Reproduce(required)
the following scenarios will cause the backup to fail, but the error messages are all:there is no meta files in backup info，the real error information needs to be viewed in the meta，I hope to print detailed error information in the br log:

• When backup, there is OFFLINE METAD, STORAGED;
• When backup, there are indexes of rebuilding;
• When backup, the specified space is not found

[kqzh]

I think this requires changes to the CreateBackup interface of the meta, when resp's meta files == empty, need return the detail err, then br-tool can print this details into log file. @shanlai