Blender Git Loki

UI: Allow translation for node error messages

This commit exposes the strings used in the node error messages for
localization. It also changes the message tooltip creation to
automatically add the period at the end, to be more consistent with
the (arguably bad) design of other tooltips in Blender.

Calling `TIP_` directly in the node implementation files allows us to
continue using `std::string` concatenation instead of passing variadic
arguments. It's also more explicit about which part of the message is
translated and which isn't. The files already include the translation
header anyway.