Handle missing OneToOneField relation when serialising FailureLines
Handle missing OneToOneField relation when serialising FailureLines

Nullable OneToOneFields raise RelatedObjectDoesNotExist when they have
no related object.  However this exception appears to be hard to catch
since its constructed at runtime.  However ObjectDoesNotExist is a
parent exception so we can still rely on that.

KWierso/treeherder,KWierso/treeherder,edmorley/treeherder,edmorley/treeherder,edmorley/treeherder,KWierso/treeherder,edmorley/treeherder,KWierso/treeherder