Add a verbose error message for BlockingError (#451)

Add a verbose error message for EnterError while trying to run
tokio_threadpool::blocking on a current_thread::Runtime

tokio-threadpool/src/blocking.rs

@@ -6,7 +6,6 @@ use std::error::Error;
 use std::fmt;
 
 /// Error raised by `blocking`.
-#[derive(Debug)]
 pub struct BlockingError {
     _p: (),
 }
@@ -156,6 +155,14 @@ impl fmt::Display for BlockingError {
     }
 }
 
+impl fmt::Debug for BlockingError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_struct("BlockingError")
+            .field("reason", &self.description())
+            .finish()
+    }
+}
+
 impl Error for BlockingError {
     fn description(&self) -> &str {
         "`blocking` annotation used from outside the context of a thread pool"