blob_size is undefined in baseblobservice

### Which service(blob, file, queue) does this issue concern?
blob

### Which version of the SDK was used? Please provide the output of `pip freeze`.
v2.1.0

### What problem was encountered?
`blob_size` is undefined in line 2212 in `azure-storage-python/azure-storage-blob/azure/storage/blob/baseblobservice.py`

### Have you found a mitigation/solution?
Maybe we also set `blob_size` in the except branch in line 2189 and so we have it defined in both cases.

Note: for table service, please post the issue here instead: https://github.com/Azure/azure-cosmosdb-python.

The bug is in azure-storage-python/azure-storage-blob/azure/storage/blob/baseblobservice.py, around lines 2189-2212. Start by tracing how blob_size is assigned in the normal and exception paths for the reported blob operation. Done means the operation no longer raises an undefined-variable error when the exception path is taken.

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