Restore qiskit.qiskiterror (#1602)

CHANGELOG.rst

@@ -20,6 +20,12 @@ The format is based on `Keep a Changelog`_.
 `UNRELEASED`_
 =============
 
+Changed
+-------
+
+- The ``Exception`` subclasses have been moved to an ``.exceptions`` module
+  within each package (for example, ``qiskit.exceptions.QiskitError``). (#1600).
+
 
 `0.7.0`_ - 2018-12-19
 =====================

qiskit/qiskiterror.py

@@ -0,0 +1,17 @@
+# -*- coding: utf-8 -*-
+
+# Copyright 2017, IBM.
+#
+# This source code is licensed under the Apache License, Version 2.0 found in
+# the LICENSE.txt file in the root directory of this source tree.
+
+"""Exception for errors raised by Qiskit.
+
+Note: this module will be deprecated in Terra 0.7+. Please import the exceptions
+from the `qiskit.exceptions` module instead.
+"""
+
+# pylint: disable=unused-import
+
+from .exceptions import QISKitError
+from .exceptions import QiskitError