[Feature request] Add a way to get the length of a PQerrorMessage in libpq

Hello hackers,

I've been using PostgreSQL for about one year, and so far I like it a
lot, way more than MySQL or MariaDB, but I still have a little feature
request for libpq.

Could you add a way to get the size of an error message in libpq?

PQerrorMessage isn't enough when you want to do exact size string
concatenation or copy of the error message.