catch exec errors

t/cmdline-invalidcommand.txtar

@@ -1,2 +1,2 @@
-exec testrpn 1 2 dumb
+! exec testrpn 1 2 dumb
 stdout 'unknown command or operator'

t/cmdline-short-stack.txtar

@@ -1,2 +1,2 @@
-exec testrpn 4 +
+! exec testrpn 4 +
 stdout 'stack doesn''t provide enough arguments'

t/cmdlinecalc-divzero.txtar

@@ -1,2 +1,2 @@
-exec testrpn 100 50 50 - /
-stdout 'division by null\n'
+! exec testrpn 100 50 50 - /
+stdout 'division by null'