Merge #18412: script: fix SCRIPT_ERR_SIG_PUSHONLY error string

Summary:
41ff4992e57f8626019c0b2ab3d024db71e4c20f script: fix SCRIPT_ERR_SIG_PUSHONLY error string (Sebastian Falbesoner)

Pull request description:

Fixes #18411, changing the error message from `"Only non-push operators allowed in signatures"` to `"Only push operators allowed in signatures"`.

ACKs for top commit:

laanwj:
  ACK 41ff4992e57f8626019c0b2ab3d024db71e4c20f

Tree-SHA512: 3b75d83e2198d638d599ef6a4a8da986f0158600fe3f89f55b3759554588157acf2b0cba3f6a907164617264e7aee727d6d460b510c8b37ca7728aa79e11ad80

Backport of Core PR18412

Although our error message is functionally correct, this brings us more inline with Core (per feedback from https://github.com/Bitcoin-ABC/bitcoin-abc/issues/393)

Test Plan: ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6696