Make plpgsql_trap test more robust and less resource-intensive.

We were using "select count(*) into x from generate_series(1,
1_000_000_000_000)" to waste one second waiting for a statement
timeout trap.  Aside from consuming CPU to little purpose, this could
easily eat several hundred MB of temporary file space, which has been
observed to cause out-of-disk-space errors in the buildfarm.
Let's just use "pg_sleep(10)", which is far less resource-intensive.

Also update the "when others" exception handler so that if it does
ever again trap an error, it will tell us what error.  The cause of
these intermittent buildfarm failures had been obscure for awhile.

Discussion: https://postgr.es/m/557992.1776779694@sss.pgh.pa.us
Backpatch-through: 14

src/pl/plpgsql/src/expected/plpgsql_trap.out

@@ -138,13 +138,11 @@ select * from foo;
 drop table foo;
 create function trap_timeout() returns void as $$
 begin
-  declare x int;
   begin
-    -- we assume this will take longer than 1 second:
-    select count(*) into x from generate_series(1, 1000000000000);
+    perform pg_sleep(10);
   exception
     when others then
-      raise notice 'caught others?';
+      raise notice 'caught others: %', sqlerrm;
     when query_canceled then
       raise notice 'nyeah nyeah, can''t stop me';
   end;
@@ -157,7 +155,7 @@ set statement_timeout to 1000;
 select trap_timeout();
 NOTICE:  nyeah nyeah, can't stop me
 ERROR:  end of function
-CONTEXT:  PL/pgSQL function trap_timeout() line 15 at RAISE
+CONTEXT:  PL/pgSQL function trap_timeout() line 13 at RAISE
 rollback;
 -- Test for pass-by-ref values being stored in proper context
 create function test_variable_storage() returns text as $$

src/pl/plpgsql/src/sql/plpgsql_trap.sql

@@ -85,13 +85,11 @@ drop table foo;
 
 create function trap_timeout() returns void as $$
 begin
-  declare x int;
   begin
-    -- we assume this will take longer than 1 second:
-    select count(*) into x from generate_series(1, 1000000000000);
+    perform pg_sleep(10);
   exception
     when others then
-      raise notice 'caught others?';
+      raise notice 'caught others: %', sqlerrm;
     when query_canceled then
       raise notice 'nyeah nyeah, can''t stop me';
   end;