Make error handling a tiny bit better

Author: jamesots

lib/src/connection.dart

@@ -212,11 +212,14 @@ class _Connection {
         }
         _completer.complete(response.result);
       }
-    } catch (e) {
+    } catch (e, st) {
       autoRelease = true;
       _finishAndReuse();
       log.fine("completing with exception: $e");
-      _completer.completeError(e);
+      if (_completer.isCompleted) {
+        throw e;
+      }
+      _completer.completeError(e, st);
     }
   }
 

lib/src/connection_pool.dart

@@ -100,11 +100,11 @@ class ConnectionPool extends Object with _ConnectionHelpers implements Queriable
       cnx.autoRelease = true;
       _log.finest("Logged in on cnx#${cnx.number}");
       c.complete(cnx);
-    } catch (e) {
+    } catch (e, st) {
       if (!(e is MySqlException)) {
         _removeConnection(cnx);
       }
-      c.completeError(e);
+      c.completeError(e, st);
     }
   }