Add PartiallyFailed phase for backups, don't exit on first error

[skriss]

rel #1371

This PR adds the PartiallyFailed phase for backups. It modifies the backup execution logic so that rather than immediately exiting when the first error is encountered, the error is logged and execution continues. At the end, the number of errors logged is counted, and a >0 result moves the backup to PartiallyFailed.

Still a WIP, but the basic functionality is working. An easy way to see what it looks like is to create a cluster role binding for a cluster role that doesn't exist, to a service account in the namespace you're backing up -- this will create a "partial failure" error.

@nrb @carlisia appreciate any input at this point!

[skriss]

output of velero backup get:

NAME             STATUS            CREATED                         EXPIRES   STORAGE LOCATION   SELECTOR
partial-fail     PartiallyFailed   2019-04-19 08:40:53 -0600 MDT   29d       default            <none>
partial-fail-2   PartiallyFailed   2019-04-19 08:52:02 -0600 MDT   29d       default            <none>

output of velero backup logs partial-fail-2 | grep error:

time="2019-04-19T14:52:16Z" level=info msg="1 errors encountered backup up item" backup=velero/partial-fail-2 group=v1 logSource="pkg/backup/resource_backupper.go:265" name=velero namespace=velero resource=serviceaccounts
time="2019-04-19T14:52:16Z" level=error msg="Error backing up item" backup=velero/partial-fail-2 error="clusterroles.rbac.authorization.k8s.io \"non-existent\" not found" error.file="/go/src/github.com/heptio/velero/pkg/backup/item_backupper.go:328" error.function="github.com/heptio/velero/pkg/backup.(*defaultItemBackupper).executeActions" group=v1 logSource="pkg/backup/resource_backupper.go:267" name=velero namespace=velero resource=serviceaccounts

[nrb]

Overall this is looking good!

[skriss]

Made a few more updates - this is ready for review. Still testing.

[carlisia]

Super minor suggestions. Besides those, I didn't see in this code any logging of warning. I suppose you are displaying warnings that we were already adding.

[skriss]

addressed comments in latest commits

[carlisia]

lgtm, just needs rebasing.

[skriss]

rebased/squashed/changelogged