R1:8bffc9ea0efe

In "bin/bulk export", require "--output <path>" by default

Summary:
Depends on D19743. Ref T13210. Since this command can easily dump a bunch of binary data (or just a huge long blob of nonsense) to stdout, default to requiring "--output <file>".

Using `--output -` will print to stdout.

Test Plan: Ran with: no `--output`, `--output file`, `--output -`, `--output - --overwrite`. Got sensible results or errors in all cases.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210…