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Return the NetworkError if the connectGATT fails.

This is the second of two patches to implement NetworkError:
  [1] https://codereview.chromium.org/1124973003
  [2] This patch.

BUG=484779

Review URL: https://codereview.chromium.org/1118053003

Cr-Commit-Position: refs/heads/master@{#329512}