R1:37b8ec5bb73e

Provide an "--output" mode for "bin/storage dump" with better error handling

Summary:
Ref T6996. If you do this kind of thing in the shell, you don't get a good error by default if the `dump` command fails:

```
$ bin/storage dump | gzip > output.sql.gz
```

This can be worked around with some elaborate bash tricks, but they're really clunky and uninintuitive.

We also need to do this in several places (while writing backups; while performing exports), and I don't want to copy clunky bash tricks all over…