Fix errors and make the migration more verbose

We had some exceptions with the first attempt to migrate the values for
vulnerabilities. These changes fixes those exceptions and add more log
messages to make it easier to check the migration result.

db/schema_migrations/20200806100713

-fdcce45050f972d8edf2c645022f517ff6b9f4c76767e6cebe45a11fe34dd388
\ No newline at end of file

db/schema_migrations/20200826220746

+2564c387b727e557b2988996aa533ba5e4e6d7b01515407bd2692c09644ac2be
\ No newline at end of file

ee/lib/ee/gitlab/background_migration/populate_resolved_on_default_branch_column.rb

@@ -90,6 +90,8 @@ module EE
           end
 
           def reports
+            return [] unless default_branch
+
             @reports ||= artifacts.to_a.map(&:reports).flatten
           end
 
@@ -345,6 +347,7 @@ module EE
 
           def root_ref
             raw_repository&.root_ref
+          rescue Gitlab::Git::Repository::NoRepository
           end
 
           def empty?
@@ -392,6 +395,10 @@ module EE
             project.vulnerabilities
                    .id_not_in(existing_vulnerability_ids)
                    .update_all(resolved_on_default_branch: true)
+
+            log_info
+          rescue StandardError => e
+            log_error(e)
           end
 
           private
@@ -400,16 +407,47 @@ module EE
 
           delegate :reports, to: :project, private: true
 
+          def log_info
+            ::Gitlab::BackgroundMigration::Logger.info(
+              migrator: 'PopulateResolvedOnDefaultBranchColumnForProject',
+              message: 'Project migrated',
+              stats: stats,
+              project_id: project_id
+            )
+          end
+
+          def stats
+            {
+              all_count: findings.length,
+              valid_count: all_valid_findings.length,
+              existing_count: existing_vulnerability_ids.length
+            }
+          end
+
+          def log_error(error)
+            ::Gitlab::BackgroundMigration::Logger.error(
+              migrator: 'PopulateResolvedOnDefaultBranchColumnForProject',
+              message: error.message,
+              project_id: project_id
+            )
+          end
+
           def project
             @project ||= Project.find(project_id)
           end
 
           def existing_vulnerability_ids
-            all_findings_with_scanner.map { |finding| find_saved_finding_for(finding)&.vulnerability_id }.compact
+            @existing_vulnerability_ids ||= all_valid_findings.map { |finding| find_saved_finding_for(finding)&.vulnerability_id }.compact
+          end
+
+          def all_valid_findings
+            @all_valid_findings ||= findings.select(&:scanner)
+                                           .select(&:primary_identifier)
+                                           .select(&:location)
           end
 
-          def all_findings_with_scanner
-            reports.flat_map(&:findings).select(&:scanner)
+          def findings
+            @findings ||= reports.flat_map(&:findings)
           end
 
           def find_saved_finding_for(finding)