Clear conn->errorMessage at successful completion of PQconnectdb().

Commits ffa2e467 and 52a10224 caused libpq's connection-establishment
functions to usually leave a nonempty string in the connection's
errorMessage buffer, even after a successful connection.  While that
was intentional on my part, more sober reflection says that it wasn't
a great idea: the string would be a bit confusing.  Also this broke at
least one application that checked for connection success by examining
the errorMessage, instead of using PQstatus() as documented.  Let's
clear the buffer at success exit, restoring the pre-v14 behavior.

Discussion: https://postgr.es/m/4170264.1620321747@sss.pgh.pa.us

src/interfaces/libpq/fe-connect.c

@@ -3658,6 +3658,13 @@ keep_going:						/* We will come back to here until there is
 				/* We can release the address list now. */
 				release_conn_addrinfo(conn);
 
+				/*
+				 * Contents of conn->errorMessage are no longer interesting
+				 * (and it seems some clients expect it to be empty after a
+				 * successful connection).
+				 */
+				resetPQExpBuffer(&conn->errorMessage);
+
 				/* We are open for business! */
 				conn->status = CONNECTION_OK;
 				return PGRES_POLLING_OK;