Description

This PR adds error logging to the bulk loader. Prior to these changes, error handling in bulk loading allowed for only ignoring errors or stopping the load upon any error. These changes allow for errors to be logged to a file so that the entire bulk loading operation does not have to be an "all or nothing" exercise -- important for loads that can take hours to complete.

The new flags look like this:

dgraph bulk .... --ignore_errors  --log_errors   --error_log=/tmp/bulktest/errors.log

The existing --ignore_errors flag is required. The location of the error log can be specfied, the default is bulk_errors.log.

Checklist

• The PR title follows the
  Conventional Commits syntax, leading
  with fix:, feat:, chore:, ci:, etc.
• Code compiles correctly and linting (via trunk) passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable
• For public APIs, new features, etc., a PR on the
  docs repo staged and linked here. This process can
  be simplified by going to the public docs site and clicking the
  "Edit this page" button at the bottom of page(s) relevant to your changes. Ensure that you
  indicate in the PR that this is an unreleased feature so that it does not get merged into
  the main docs prematurely.