Formatting in error messages

[Jomy10]

Currently, error messages raised from Ruby aren't very helpful.

Example

use rutie::VM;

fn main() {
    VM::init();

    VM::eval("a()").unwrap();
}

Running this program will output:

thread 'main' panicked at src/main.rs:6:21:
called `Result::unwrap()` on an `Err` value: #<NoMethodError: undefined method '#<Symbol:0x000000000075010c>' for main>

It seems that #<Symbol:0x000000000075010c> representing a is not being converted to a string properly.

I've done some minimal searching, but couldn't determine where this string originates from.

If you could give me some pointers on where to fix this, I'd gladly open a pull-request, or if you have time to fix the issue.