fix error when finding relative paths on windows.

io_import_images_as_planes.py

@@ -453,7 +453,15 @@ class IMPORT_OT_image_to_plane(Operator, AddObjectHelper):
         image.use_fields = self.use_fields
 
         if self.relative:
-            image.filepath = bpy.path.relpath(image.filepath)
+            # can't always find the relative path
+            # (between drive letters on windows)
+            try:
+                filepath_rel = bpy.path.relpath(image.filepath)
+            except ValueError:
+                filepath_rel = None
+
+            if filepath_rel is not None:
+                image.filepath = filepath_rel
 
     def set_texture_options(self, context, texture):
         texture.image.use_alpha = self.use_transparency