The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/10/static/contrib-dblink-error-message.html
Description:
Documentation says:
Return Value
Returns last error message, or an empty string if there has been no error in
this connection.
Which is invalid.
Actually it returns 'OK' string if no error was raised. Secondly
dblink_is_busy must be first called to make dblink_error_message returns an
error message. (Tested on 9.6.9)