ubuntu_vfat_stress will pass even with error reported by stress-ng

The test will pass even with stress-ng complaining about "No space left on device", not sure if this is an expected behaviour, but it might be confusing to testers.

 stress-ng: info: [36588] dispatching hogs: 2 hdd, 2 lockf, 2 seek, 2 aio, 2 dentry, 2 dir, 2 fallocate, 2 fstat, 2 lease, 2 open, 2 rename, 2 chdir, 2 rename
 stress-ng: fail: [36609] stress-ng-chdir: mkdir failed, errno=28 (No space left on device)
 stress-ng: fail: [36623] stress-ng-chdir: mkdir failed, errno=28 (No space left on device)
 stress-ng: fail: [36611] stress-ng-hdd: read failed, errno=28 (No space left on device)
 stress-ng: error: [36588] process 36611 (stress-ng-hdd) terminated with an error, exit status=1 (stress-ng core failure)
 stress-ng: error: [36588] process [36609] (stress-ng-chdir) aborted early, out of system resources
 stress-ng: error: [36588] process [36623] (stress-ng-chdir) aborted early, out of system resources
 stress-ng: info: [36588] unsuccessful run completed in 14.35s
 stress-ng: info: [36588] stressor bogo ops real time usr time sys time bogo ops/s bogo ops/s
 stress-ng: info: [36588] (secs) (secs) (secs) (real time) (usr+sys time)
 stress-ng: info: [36588] hdd 16391 7.39 0.44 6.44 2218.21 2382.41
 stress-ng: info: [36588] lockf 131604 10.16 0.05 12.43 12947.00 10545.19
 stress-ng: info: [36588] seek 1987284 10.17 1.33 10.40 195448.06 169418.93
 stress-ng: info: [36588] aio 2496 10.15 0.00 0.00 245.83 0.00
 stress-ng: info: [36588] dentry 171 10.42 0.00 0.61 16.41 280.33
 stress-ng: info: [36588] dir 2539 14.09 0.00 15.40 180.14 164.87
 stress-ng: info: [36588] fallocate 170 10.17 0.04 5.35 16.71 31.54
 stress-ng: info: [36588] fstat 2147 10.00 0.02 0.26 214.64 7667.86
 stress-ng: info: [36588] lease 66 10.14 0.61 5.15 6.51 11.46
 stress-ng: info: [36588] open 1434208 10.11 0.44 12.20 141807.28 113465.82
 stress-ng: info: [36588] rename 6526 10.01 0.00 2.94 651.86 2219.73
 stress-ng: info: [36588] rename 6820 10.00 0.00 2.49 681.85 2738.96
 stress-ng: info: [36588] for a 14.35s run time:
 stress-ng: info: [36588] 114.80s available CPU time
 stress-ng: info: [36588] 3.05s user time ( 2.66%)
 stress-ng: info: [36588] 75.50s system time ( 65.77%)
 stress-ng: info: [36588] 78.55s total time ( 68.43%)
 stress-ng: info: [36588] load average: 17.49 10.98 4.90
 Stress-ng stressor failed, error: 2
 umounting vfat /mnt/vfat-test-29590
 umounting tmpfs /mnt/vfat-test-29590
 ================================================================================

 Completed

 Kernel issues: NONE

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.4.0-139-generic 4.4.0-139.165
ProcVersionSignature: Ubuntu 4.4.0-139.165-generic 4.4.160
Uname: Linux 4.4.0-139-generic s390x
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
AlsaDevices: Error: command ['ls', '-l', '/dev/snd/'] failed with exit code 2: ls: cannot access '/dev/snd/': No such file or directory
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: s390x
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
CRDA: Error: command ['iw', 'reg', 'get'] failed with exit code 1: nl80211 not found.
CurrentDmesg:

Date: Fri Nov 2 08:20:40 2018
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
Lspci:

Lsusb: Error: command ['lsusb'] failed with exit code 1:
PciMultimedia:

ProcFB: Error: [Errno 2] No such file or directory: '/proc/fb'
ProcKernelCmdLine: root=UUID=44b0b919-a1a4-4849-9425-e71d4ac87d85 crashkernel=196M nobp=1 BOOT_IMAGE=0
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-139-generic N/A
 linux-backports-modules-4.4.0-139-generic N/A
 linux-firmware 1.157.20
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)