Missing exceptions module error causing Flask-Restless to break

The exceptions module was removed from the latest version of Flask
(0.9) and since it was added in the same version, it doesn't show up
in any earlier versions. This means that Flask-Restless is broken for
all versions of Flask. This commit adds the JSONHTTPResponse from the
removed exceptions module in Flask to the exceptions module in
Flask-Restless. The commit where the exceptions module was removed can
be seen here:
https://github.com/mitsuhiko/flask/commit/4c27f7a8c4bbe6621681178be716e6270067a3ad.