Unclear error message for invalid string

New users may not know that `$` (or some other character) has a special meaning in Julia, in which case this error is confusing:
```julia
julia> str = "hello $x$"
ERROR: ParseError:
# Error @ REPL[26]:1:17
str = "hello $x$"
#               └ ── identifier or parenthesized expression expected after $ in string
```