Add Lua runtime helper functions for error-handling

Add two helper functions to the Lua runtime, "SWIG_Lua_pusherrstring"
and "SWIG_Lua_pushferrstring".

These are like the standard Lua functions lua_pushstring and
lua_pushfstring respectively, except that the strings are prefixed
with the location of the innermost Lua call-point (as generated by
luaL_where).
Include Lua error locus in SWIG error messages

This is standard information in Lua error messages, and makes it much
easier to find bugs.
Document Include Lua error locus in SWIG error messages patches.

Closes #81