Simplicity needs this fix.

git-subtree-dir: src/simplicity
git-subtree-split: 86ac0f92c4b2b6d694dca85dce3b6909aed25894

also filters src/simplicity/secp256k1 from the coverage results

This will be used by Simplicity.

Take the existing function to parse script errors and extend it to parse
Elements errors. SCRIPT_ERR_ERROR_COUNT is not included because it is a
pseudo error.

Compare the expected script error, parsed from JSON, with the actual
error that VerifyScript returns. If the JSON does not include an
expected error, then skip this check.