Don't issue a "disk full" error if we couldn't determine disk free space

Summary: Sometimes this will fail, e.g. because 'open_basedir' restricts us.
Fall through to the more generic error message if we failed to determine disk
free space for any reason.
Test Plan: When configured with a restrictive 'open_basedir', I got a more
accurate exception instead of being told the disk was full.
Reviewed By: jungejason
Reviewers: 10098, jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 718