Error handling in C API function calls in a way that doesn't close client connection

Hello all,

This question refers to version 9.4 of Postgres. I have have a function

Datum do_something(PG_FUNCTION_ARGS)
{
...
if(error_occured) {
ereport(ERROR, (errmsg("some error occured")));
}
...
}

When I call do_something in a way to deliberately cause the error
condition, the client connection (psql) is terminated, however for example
when you say (or many other function calls that cause ereport to be called):

psql > SELECT ST_IsSimple('POINT(1, 1)');

I simply get the message "ERROR: parse error - invalid geometry" but the
connection is not closed.

The subqueries also know that the inner query had terminated with an error
somehow, without terminating the connection:

psql> select * from (select st_issimple('point(0, 0)')) as a;

ERROR: parse error - invalid geometry
I guess this could be done with surrounding the function call with
PG_TRY/CATCH however these macros are not present in Postgis code.

Can you please suggest how to implement this functionality in my code?

Thanks,
Igor