Cherry-pick changes from rlua:

- Make Value::type_name() public
- Update CallbackError and ExternalError Display impl

src/error.rs

@@ -184,10 +184,10 @@ impl fmt::Display for Error {
             Error::MismatchedRegistryKey => {
                 write!(fmt, "RegistryKey used from different Lua state")
             }
-            Error::CallbackError { ref traceback, ref cause } => {
-                write!(fmt, "callback error: {}: {}", cause, traceback)
+            Error::CallbackError { ref traceback, .. } => {
+                write!(fmt, "callback error: {}", traceback)
             }
-            Error::ExternalError(ref err) => write!(fmt, "external error: {}", err),
+            Error::ExternalError(ref err) => write!(fmt, "{}", err),
         }
     }
 }
@@ -196,7 +196,7 @@ impl StdError for Error {
     fn source(&self) -> Option<&(dyn StdError + 'static)> {
         match *self {
             Error::CallbackError { ref cause, .. } => Some(cause.as_ref()),
-            Error::ExternalError(ref err) => Some(err.as_ref()),
+            Error::ExternalError(ref err) => err.source(),
             _ => None,
         }
     }

src/value.rs

@@ -48,18 +48,19 @@ pub enum Value<'lua> {
 pub use self::Value::Nil;
 
 impl<'lua> Value<'lua> {
-    pub(crate) fn type_name(&self) -> &'static str {
+    pub fn type_name(&self) -> &'static str {
         match *self {
             Value::Nil => "nil",
             Value::Boolean(_) => "boolean",
-            Value::LightUserData(_) => "light userdata",
+            Value::LightUserData(_) => "lightuserdata",
             Value::Integer(_) => "integer",
             Value::Number(_) => "number",
             Value::String(_) => "string",
             Value::Table(_) => "table",
             Value::Function(_) => "function",
             Value::Thread(_) => "thread",
-            Value::UserData(_) | Value::Error(_) => "userdata",
+            Value::UserData(_) => "userdata",
+            Value::Error(_) => "error",
         }
     }