In PL/Tcl tests, don't choke if optional error fields are missing.

This fixes a portability issue introduced by commit 961bed02: with a
compiler that doesn't support PG_FUNCNAME_MACRO, the "funcname" field of
errorCode won't be provided, leading to a failure of the unset command.
I added -nocomplain to the unset commands for filename and lineno too, just
in case, though I know of no platform that wouldn't populate those fields.
(BTW, -nocomplain is new in Tcl 8.4, but fortunately we dropped support
for pre-8.4 Tcl some time ago.)

Per buildfarm member pademelon.

src/pl/tcl/expected/pltcl_setup.out

@@ -608,9 +608,9 @@ as $function$
         # Get rid of keys that can't be expected to remain constant
         array set myArray $::errorCode
         unset myArray(POSTGRES)
-        unset myArray(funcname)
-        unset myArray(filename)
-        unset myArray(lineno)
+        unset -nocomplain myArray(funcname)
+        unset -nocomplain myArray(filename)
+        unset -nocomplain myArray(lineno)
 
         # Format into something nicer
         set vals []

src/pl/tcl/sql/pltcl_setup.sql

@@ -653,9 +653,9 @@ as $function$
         # Get rid of keys that can't be expected to remain constant
         array set myArray $::errorCode
         unset myArray(POSTGRES)
-        unset myArray(funcname)
-        unset myArray(filename)
-        unset myArray(lineno)
+        unset -nocomplain myArray(funcname)
+        unset -nocomplain myArray(filename)
+        unset -nocomplain myArray(lineno)
 
         # Format into something nicer
         set vals []