Fix get_path() error when calling get_node()

[timothyqiu]

When failed, get_node() shows the absolute path of current node by calling get_path(). But get_path() requires the node to be inside the tree. So calling get_node("Not Exists") on a outside-tree node produces error messages like this:

ERROR: Cannot get path of node as it is not in a scene tree.
at: get_path (scene/main/node.cpp:1704)
ERROR: Node not found: "Not Exists" (relative to "").
at: get_node (scene/main/node.cpp:1348)

This PR fixes the extra error message issue and avoids an empty description of the node (relative to "").

[mhilbrunner]

Approved iny today's PR meeting

[akien-mga]

Thanks!