Abort when DISPLAY isn't valid

There's not much else to do. There's no return and making the
RenderWindow a zombie object isn't any better. Throwing an exception
isn't good either since nothing else throws exceptions around here.

Change-Id: I4cd36c6ba833f02471764b18e7700796735f6a7a
restore original settings after painting tooltip

This commit fixes a bug that affected the labels that indicate how
many vertices are hidden within a collapsed subtree.  The size and
orientation of these labels would potentially change based on whether
or not a vtkToolTipItem was currently displayed.

The problem was that vtkToolTipItem modified the vtkPen, vtkBrush, and
vtkTextProperty used by the painter.  This commit changes this behavior,
so that the original painter settings are restored after the tooltip is
drawn.  This makes the appearance of the rendered scene less dependent
on the order in which the items are painted.

Change-Id: I48acba6313a63539a2116cd8d2929fe8ab162de8
new baseline for TestColumnTree

The previous commit (9c5d606) changed the orientation of the
collapsed subtree label in this test.  While the prior orientation
also looked nice, it was a side-effect of the orientation of the
heatmap's column label.  Thus, it was subject to change based on
what order the items in the scene were rendered.  It also did not
look correct for all possible dendrogram orientations.  This new
baseline depicts the label where its orientation is determined
by that of the dendrogram.

Change-Id: Ia6ef68ff573234f1e189a3124bc92295ac85ba6a
Merge topic 'fix_label_position' into master

7e18b302 new baseline for TestColumnTree
9c5d6061 restore original settings after painting tooltip
Merge topic 'fail-gracefully-on-X-errors' into master

1d1fb483 Abort when DISPLAY isn't valid