refactor(lib): convert to futures 0.2.0-beta (#1470)

src/error.rs

@@ -21,7 +21,8 @@ use self::Error::{
     Io,
     TooLarge,
     Incomplete,
-    Utf8
+    Utf8,
+    Executor
 };
 
 /// Result type often returned from methods that can have hyper `Error`s.
@@ -56,6 +57,8 @@ pub enum Error {
     Io(IoError),
     /// Parsing a field as string failed
     Utf8(Utf8Error),
+    /// Executing a future failed
+    Executor,
 
     #[doc(hidden)]
     __Nonexhaustive(Void)
@@ -127,6 +130,7 @@ impl StdError for Error {
             Cancel(ref e) => e.description(),
             Io(ref e) => e.description(),
             Utf8(ref e) => e.description(),
+            Executor => "executor is missing or failed to spawn",
             Error::__Nonexhaustive(..) =>  unreachable!(),
         }
     }