Standardize unimplemented API failures

crates/libremetaverse/src/lib.rs

@@ -6,6 +6,7 @@ pub use generated::*;
 pub use libremetaverse_imaging as imaging_abstractions;
 pub use libremetaverse_structured_data as structured_data;
 pub use libremetaverse_types as types;
+pub use libremetaverse_types::NotImplemented as Error;
 
 /// Common behavior for program shims while the network stack is unimplemented.
 pub mod shim {