[BEAM-2808] Improve error message when DoFn @ProcessElement has the wrong window type

[youngoli]

Modifying the error message from this:

expects window type , which is not a supertype of actual window type

to this:

unable to provide window -- expected window type from parameter () is not a supertype of actual window type assigned by windowing ()

@lukecwik, can you review?

[lukecwik]

LGTM

[lukecwik]

Note that most people automatically have the standard template and keep it part of the review.
See #3856 as an example.

We also took on the convention of asking for a specific reviewer with
"R: @username" and to bring attention to this PR for a specific user with "CC: @username"
This convention has not been codified within the contribution guide and would be useful to add there if you don't mind.

[coveralls]

Coverage remained the same at 69.5% when pulling dea7271 on youngoli:bugfix into 979923a on apache:master.

[lukecwik]

Test failed, seems like you missed updating a test expectation.

[coveralls]

Coverage increased (+0.01%) to 69.514% when pulling 5980ba2 on youngoli:bugfix into 979923a on apache:master.