BulkImports: Add pipeline step to the failures log

What does this MR do?

For better understanding of BulkImport Pipeline errors, now the pipeline step (extractor, transformer, loader) will also be logged in the bulk_imports_failures table.

Related to: #299528 (closed)

rails db:migrate:up

$ r db:migrate:up VERSION=20210122155158
== 20210122155158 AddPipelineStepToBulkImportsFailures: migrating =============
-- add_column(:bulk_import_failures, :pipeline_step, :text)
   -> 0.0028s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE bulk_import_failures\nADD CONSTRAINT check_721a422375\nCHECK ( char_length(pipeline_step) <= 255 )\nNOT VALID;\n")
   -> 0.0044s
-- current_schema()
   -> 0.0003s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- execute("ALTER TABLE bulk_import_failures VALIDATE CONSTRAINT check_721a422375;")
   -> 0.0017s
-- execute("RESET ALL")
   -> 0.0002s
== 20210122155158 AddPipelineStepToBulkImportsFailures: migrated (0.0192s) ====

rails db:migrate:down

$ r db:migrate:down VERSION=20210122155158
== 20210122155158 AddPipelineStepToBulkImportsFailures: reverting =============
-- remove_column(:bulk_import_failures, :pipeline_step)
   -> 0.0124s
== 20210122155158 AddPipelineStepToBulkImportsFailures: reverted (0.0205s) ====

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team