[2.3] Unhandled error in deferred when tgt fails to start

[Andres Rodriguez]

The snap sometimes fails to start tgt, but this seems to be crashing twisted and the error is not handled properly.

2017-11-29 00:37:13 twisted.internet.defer: [critical] Unhandled error in Deferred:
2017-11-29 00:37:13 twisted.internet.defer: [critical]

Traceback (most recent call last):
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 434, in errback
    self._startRunCallbacks(fail)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
    self._runCallbacks()
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 588, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1184, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/snap/maas/1315/lib/python3.5/site-packages/provisioningserver/rpc/boot_images.py", line 145, in _import_boot_images
    imported = yield deferToThread(_run_import, sources, **proxies)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/snap/maas/1315/lib/python3.5/site-packages/provisioningserver/utils/twisted.py", line 232, in wrapper
    result = func(*args, **kwargs)
  File "/snap/maas/1315/lib/python3.5/site-packages/provisioningserver/rpc/boot_images.py", line 119, in _run_import
    imported = boot_resources.import_images(sources)
  File "/snap/maas/1315/lib/python3.5/site-packages/provisioningserver/import_images/boot_resources.py", line 310, in import_images
    update_iscsi_targets(os.path.join(storage, 'current'))
  File "/snap/maas/1315/lib/python3.5/site-packages/provisioningserver/import_images/boot_resources.py", line 258, in update_iscsi_targets
    update_targets_conf(snapshot_path)
  File "/snap/maas/1315/lib/python3.5/site-packages/provisioningserver/import_images/boot_resources.py", line 208, in update_targets_conf
    service_monitor.ensureService("tgt").wait(30)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/crochet/_eventloop.py", line 231, in wait
    result.raiseException()
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/python/failure.py", line 368, in raiseException
    raise self.value.with_traceback(self.tb)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/snap/maas/1315/lib/python3.5/site-packages/provisioningserver/utils/service_monitor.py", line 567, in _ensureService
    yield self._performServiceAction(service, action)
  File "/snap/maas/1315/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1128, in _inlineCallbacks
    result = g.send(result)
  File "/snap/maas/1315/lib/python3.5/site-packages/provisioningserver/utils/service_monitor.py", line 422, in _performServiceAction
    raise ServiceActionError(error_msg)
provisioningserver.utils.service_monitor.ServiceActionError: Service 'tgt' failed to start: