[Lua] Add error-locus information to SWIG error messages

Standard Lua error messages include the source location of the innermost Lua call that resulted in the error, which makes debugging much easier.

This change adds the same information to errors raised from SWIG, using the standard Lua "luaL_where" function to identify and format the location.

One notable exception is errors resulting from exceptions, where the string seems to be content rather than informative.