Ensure bufferWriter is always closed in Migration.Buffer and propagate close errors (#1308)

* Ensure bufferWriter is always closed and error is returned in Buffer()

* Ensure bufferWriter is always closed and error is returned in Buffer()

* addressed review comments. used named error and joined error to return final error

* Fixed defer block

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Co-authored-by: Chandra kant <chandra.kant@cohesity.com>