This is in preparation for the next commit, which removes the useless
`ret` parameter from ValidationState::Invalid().
error() is simply a convenience wrapper that calls LogPrintf and returns
false. Call LogPrintf explicitly and substitute the error() call for a
false bool literal.
Partial backport of Core PR15921 : https://github.com/bitcoin/bitcoin/pull/15921/commits/1a37de4b3174d19a6d8691ae07e92b32fdfaef11