bug: Close bolt on error when updating index

[CascadingRadium]

• When updating an index with an updated mapping, we open bolt via scorch's OpenMeta API but we never close it on error.
• The error case can happen if the user has given an invalid mapping or if the updated mapping does not satisfy the criterion required for index update.
• Scorch's Close() API handles the dual case:
  • If only bolt is opened but not scorch, it closes only bolt.
  • If both bolt and scorch is opened, it closes both.
• Fix the issue by always closing scorch on error, irrespective of whether it was opened using OpenMeta or Open or OpenMeta + Open.

[coveralls]

coverage: 52.339% (-0.01%) from 52.351% — boltLeak into master