Introduce option to ignore error when blob to snapshot does not exist

[udf2457]

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At the present time, running azure storage blob snapshot against a blob that does not exist yields the following error:

error:   Blob test in Container data doesn't exist
error:   Error information has been recorded to /home/bp/.azure/azure.err
error:   storage blob snapshot command failed

It would be useful to be able to ignore this error condition through an option.

Example use-case:

Imagine you are writing a shell script to upload to Azure. It would be useful to have a "snapshot if exists" option for obvious reasons.

[balajikris]

@hasonmsft - can you please take a look?

[hasonmsft]

@udf2457 , thanks for your feedback. We'll put it into our backlog and triage it.