# Steps to reproduce

* Start `localstack` (version 0.10.3).
* Don't create any lambda functions.
* Run:

```sh
AWS_ACCESS_KEY_ID='a' AWS_SECRET_ACCESS_KEY='b' \
aws lambda invoke \
  --function-name 'my-func' \
  --qualifier='qual' \
  --endpoint http://localhost:4574 test.out
```

* Expected behaviour — a `ResourceNotFoundException` from `aws-cli`.
* Actual behaviour — `An error occurred (500) when calling the Invoke operation` message and a 500 response.

This bug only appears if a `qualifier` is included in the request.
if isinstance(e, ClientError):
            return e.get_response()
        return error_response('Unknown error: %s %s' % (e, traceback.format_exc()))
@app.route('%s/functions/<function>' % PATH_ROOT, methods=['GET'])
def get_function(function):
    """ Get details for a single function
        ---
        operationId: 'getFunction'
        parameters:
            - name: 'request'
              in: body
            - name: 'function'
              in: path
    """
    funcs = do_list_functions()
    for func in funcs:
        if func['FunctionName'] == function:
            result = {
                'Configuration': func,
                'Code': {
                    'Location': '%s/code' % request.url
                }
            }
            lambda_details = arn_to_lambda.get(func['FunctionArn'])
            if lambda_details.concurrency is not None:
                result['Concurrency'] = lambda_details.concurrency
            return jsonify(result)
    return not_found_error(func_arn(function))
@app.route('%s/functions/' % PATH_ROOT, methods=['GET'])
def list_functions():