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.

Signed-off-by: Fedor Indutny <fedor@indutny.com>