[MD Extensions] Include the manifest code snippet in load errors

Add the manifest code snippet to errors displayed when loading an
unpacked extension fails. Add tests for the same. Update Service to
display the load error dialog when a load error occurs.

BUG=529395
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2769293002
Cr-Commit-Position: refs/heads/master@{#459636}