Improve VFS error handling and log config name in VFS error flow

[dulanjalidilmi]

Purpose

Improve VFS error handling and log config name in VFS error flow
Fix wso2/api-manager#1945

Port: https://github.com/wso2-support/wso2-synapse/pull/2136 and https://github.com/wso2-support/wso2-synapse/pull/2161

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