Remove liblo_error error code

Instead define a libloadorder_category error category to identify
libloadorder errors. This allows the libloadorder error code to be
preserved, and is more standard-c++-like.

docs/api/reference.rst

@@ -48,3 +48,11 @@ Classes
 
 .. doxygenclass:: loot::LootVersion
    :members:
+
+Error Categories
+================
+
+LOOT uses error category objects to identify errors with codes that originate in
+lower-level libraries.
+
+.. doxygenfunction:: loot::libloadorder_category