Add ToString metamethod, make LuaError implement LuaUserData

LuaError implementing LuaUserData makes it easy to return LuaResult from a
callback to implement callback functions that can error.

src/conversion.rs

@@ -1,5 +1,6 @@
 use std::collections::{HashMap, BTreeMap};
 use std::hash::Hash;
+use error_chain::ChainedError;
 
 use error::*;
 use lua::*;
@@ -263,3 +264,15 @@ impl<'lua, T: FromLua<'lua>> FromLua<'lua> for Option<T> {
         }
     }
 }
+
+impl LuaUserDataType for LuaError {
+    fn add_methods(methods: &mut LuaUserDataMethods<Self>) {
+        methods.add_method("backtrace", |lua, err, _| {
+            lua.pack(format!("{}", err.display()))
+        });
+
+        methods.add_meta_method(LuaMetaMethod::ToString, |lua, err, _| {
+            lua.pack(err.to_string())
+        });
+    }
+}

src/lua.rs

@@ -748,6 +748,8 @@ pub enum LuaMetaMethod {
     NewIndex,
     /// The call "operator" `obj(arg1, args2, ...)`.
     Call,
+    /// tostring(ud) will call this if it exists
+    ToString,
 }
 
 /// Stores methods of a userdata object.
@@ -1571,6 +1573,7 @@ impl Lua {
                                 LuaMetaMethod::Index => "__index",
                                 LuaMetaMethod::NewIndex => "__newIndex",
                                 LuaMetaMethod::Call => "__call",
+                                LuaMetaMethod::ToString => "__tostring",
                             };
                             push_string(self.state, name);
                             self.push_value(