Fix occasional error display bugs

[Quail-Language]

From time to time instead of outputting the erroneous line, interpreter outputs following:

!!! internal error !!! unable to display line !!!

E.g. with this code:

f = x -> print(x)
f("Hello")

I get:

Parser error:
At character 6 line 1
 1 | !!! internal error !!! unable to display line !!!
     ^~
Unparseable expression or statement