Returns a string containing the last error, or NULL if no error has occurred.
If a link
resource is passed to
ingres_error() it returns the last error recorded for
the link. If no link is passed then ingres_error()
returns the last error reported using the default link.
The function, ingres_error(), should always be called after executing any database query. Calling another function before ingres_error() is called will reset or change any error message from the last Ingres function call.