Bulk Importer: Do not stop on first error when validating an import file

The bulk importer currently immediately exits when it is validating an import file and the first error is found.

While this is helpful during actual application of the file it is definitely not helpful when running the validation. As part of this ticket, when the import process gets an error, we should check if it's being run with the dryRun flag enabled, and if so, the process should only print the error and keep going, so all the errors get printed before the validation ends.

The bulk import is called as part of the cmd/mattermost/commands/import.go file, in the bulkImportCmdF function, and its entrypoint in the application layer is the BulkImport function in the app/import.go file.

If you have questions about the ticket or you need any help, please contact miguel.delacruz in https://community.mattermost.com/ or post your question in the Community Developers channel.

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JIRA: https://mattermost.atlassian.net/browse/MM-29287