Wire up error handlers.

Replace some 400s with more appropriate 500s.

DAO methods that cause unexpected exceptions get caught and
logged by errors.py 500 error handler.

app/user/rest.py

@@ -22,6 +22,9 @@ from app import api_user
 
 user = Blueprint('user', __name__)
 
+from app.errors import register_errors
+register_errors(user)
+
 
 @user.route('', methods=['POST'])
 def create_user():