libpq error message refactoring

libpq now contains a mix of error message strings that end with
newlines and don't end with newlines, due to some newer code paths
with new ways of passing errors around.  This leads to confusion and
mistakes both during development and translation.

This adds new functions libpq_append_error() and
libpq_append_conn_error() that encapsulate common code paths for
producing error message strings.  Notably, these functions append the
newline, so that the string appearing in the code does not end with a
newline.  This makes (almost) all error message strings in libpq
uniform in this regard (and also consistent with how we handle it
outside of libpq code).  (There are a few exceptions that are
difficult to fit into this scheme, but they are only a few.)

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://www.postgresql.org/message-id/flat/7c0232ef-7b44-68db-599d-b327d0640a77@enterprisedb.com
