Mismatched pipelines give unclear error

[kennknowles]

Mistakenly mixing two pipeline gives an unclear error. This is an error, however we should improve the error message.

This could be reproduced by trying to flatten two things from different pipelines.

Improve the message for this assert:
https://github.com/aaltay/incubator-beam/blob/python-sdk/sdks/python/apache_beam/transforms/util.py#L135

Imported from Jira BEAM-526. Original Jira may contain additional context.
Reported by: altay.

[zhenglin-charlie-li]

.take-issue