Merge #14307: Consolidate redundant implementations of ParseHashStr

Summary:
9c5af58d51 Consolidate redundant implementations of ParseHashStr (Ben Woosley)

Pull request description:

This change:
* adds a length check to all calls to `ParseHashStr`, appropriate given its use to populate
  a 256-bit number from a hex str
* allows the caller to handle the failure, which allows for the more
  appropriate `JSONRPCError` on failure in `prioritisetransaction` rpc

Relative to #14288

Tree-SHA512: baa791147e5ceb3c30c70df3981aaf807bf7d4a90a0be3625540b59aa4b9a9d303a452bfef18bf167cbb833ef9591b4ef5948bf4a1ce67b421d804ae8d20ea53

Backport of Core PR14307
https://github.com/bitcoin/bitcoin/pull/14307/

Depends on D5294

Test Plan:

ninja check
ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5295