rpc generate: print useful help and error message

Summary:
PR rationale:

This was a requested follow-up to #19133 and #17700 to alleviate confusion and head-scratching by people following tutorials that use generate.

test: consider generate covered in _get_uncovered_rpc_commands()

rpc: print useful help and error message for generate

test: add rpc_generate functional test

This is a backport of core#19455

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10091