Release v0.2.1: add error messages for missing nodes, and beautify logging format

Add error messages for missing nodes, and beautify logging format.

Release v0.2.0: support emit to or fetch from specifc node(s) by string name(s)

0.2.0

• support emit to or fetch from specifc node(s) by string name(s)

Release v0.1.2: fix Context.fetch bug

fix the bug that Context.fetch may raise asyncio.QueueEmpty exception in v0.1.1