While a lambda is still processing (archive unpacking etc), the lambda state should be set to "Pending" (currently "Active") and invocations to that lambda should fail (currently, it can end up in a inconsistent state in this case, like lambdas being executed without layers).

Lambda state should reflect the current internal state, and invocations should fail if not active.
"""Get an existing event source mapping
    ---
    operationId: 'getEventSourceMapping'
    parameters:
        - name: 'request'
          in: body
    """
    region = LambdaRegion.get()
    mappings = region.event_source_mappings
    mappings = [m for m in mappings if mapping_uuid == m.get("UUID")]
    if len(mappings) == 0:
        return not_found_error()
    return jsonify(mappings[0])
@app.route("%s/event-source-mappings" % API_PATH_ROOT, methods=["POST"])
def create_event_source_mapping():