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 the codebase.

Instead, provide --output and --compress flags which just do this processing inside bin/storage dump. It will fail appropriately if any of the underlying operations fail. This also makes the write a little safer (refuses to overwrite) and the code more reusable.

Test Plan:

• Did three dumps, with no flags, --output, and --output --compress.
• Verified all three took similar amounts of time and were identical except for "Date Exported" timestamps in comments (except that the compressed one was compressed).
• Used gunzip to examine the compressed one, verified it was really compressed.
• Faked a write error, saw properly command behavior (clean up file + exit with error).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6996

Differential Revision: https://secure.phabricator.com/D16407