repl: fix multi-line input

The refactor in 3ae0b17c broke the multiline input's visual appearence.
While actually switching to this mode, the `...` prefix is not
displayed.

Additionally, account only SyntaxErrors that are happening at the parse
time, everything else should not be switching repl to the multiline
mode.

NOTE: We broke repl in a hard way, really. Let's fix it!