How to handle error message in PG_CATCH

I'm working on implementing pg_check functions. Because I want to test
whole table I need catch a error and handle it myself. I use following
construct:

PG_TRY();
{
...
ereport(ERROR, (errmsg("Error test")));
...
}
PG_CATCH();
{
errcontext("Context error");
EmitErrorReport();
FlushErrorState();
}
PG_END_TRY();

At the end I got following message:

ERROR: Error test
CONTEXT: Context error
server sent data ("D" message) without prior row description ("T" message)

and also nothing appears in a log file. Similar concept is used in
autovacuum.c.

Any idea what is wrong?

Thanks for help Zdenek