How do I check for errors returned from a C function?

[John Klimek <jklimek@...>]

For my game I plan to have lots of objects running scripts (players,
npcs, rooms, items, etc).  Whenever an object runs a script I create a
global lua userdata called "Entity".  The lua script can then check
for the actual entity type by using Entity:GetType.

Now suppose that only player entities can use the SendText C function.
 The following code would be invalid:

if (Entity:GetType() == 'item') then
  Entity:SendText("Hello!"); // invalid, item Entity's cannot use SendText
end;

So what I'd like to do is for the C function SendText is throw back an
error.  I'm aware that I can push the error message to the stack and
then use lua_error but how do I check for the error inside of my lua
script?

Also, does my above script engine logic seem OK?