Avoid UBSan warning in ProcessMessage(...) and disallow negative mocktime

Summary:
This is a backport of core#21043

Most of the work was already done in D6022. The functional test already exists in abc_rpc_mocktime.py, only the error messages need to be updated.

Test Plan:

ninja all check-all
src/bitcoin-cli help setmocktime

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11788