Syntax errors should be shown after stack

[louisrubet]

otherwise you cannot see the lexer error.
To reproduce:

rpn> 1 1000 for i i next
(...)
2> 999
1> 1000

Here you see 1000 lines of numbers (this is correct).

rpn> 1 2 start
missing next or step after for or start: error 11: syntax error
2> 999
1> 1000

Here you see a syntax error message then 1000 lines of numbers, which is not convenient.