Check that emitters passed in are lists/iterables in scheduler

[itsdawei]

Description

A potentially common error is for users to pass in a single emitter and receive a confusing error message about the emitter not having the method __len__. This PR makes the error message more clear.

TODO

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go