Balance data proceeds when there are running br backup job

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Describe the bug (required)

1 We first start a br backup, and before it finish
2. we start a balance data job migrating data from to another two new nodes

We expect the balance data jobs to report an error before start, but it did proceeds, the backup data might be inconsistent in this situation.

Your Environments (required)

• OS: uname -a
• Compiler: g++ --version or clang++ --version
• CPU: lscpu
• Commit id (e.g. a3ffc7d8) https://github.com/vesoft-inc/nebula-ent/commit/af513f9621154870e02ee865e67bf5bed5326b77 release-3.3

How To Reproduce(required)

Steps to reproduce the behavior:

1. Step 1
2. Step 2
3. Step 3

Expected behavior

Additional context