Need better error message when invalid escape sequence present

[maowtm]

a :- run("aaaaa\1bbbbbb").

Results in

error: expected "\""
  ┌─ Modusfile:1:11
  │
1 │ a :- run("aaaaa\1bbbbbb").
  │ ----------^
  │ │    │   │ 
  │ │    │   in section "modus_const"
  │ │    in section "rule_body"
  │ in section "rule"

This become very unclear if the string is very long (like a multi-line run command)

[thevirtuoso1973]

Now it points to the problematic escape char at least. I think we can still improve by printing the possible escape chars, so I'll leave this issue open for now.