Allow viewer package to import without Qt

skimage/viewer/__init__.py

@@ -1,6 +1,5 @@
+import warnings
 try:
-    from .qt import QtGui as _QtGui
+    from .viewers import ImageViewer, CollectionViewer
 except ImportError as e:
-    raise ImportError('Viewer requires Qt')
-
-from .viewers import ImageViewer, CollectionViewer
+    warnings.warn('Viewer requires Qt')