[BUG] Un-declared variables cause not graceful error messages

[Pillber]

Describe the bug
When using a variable that has not been declared using let, instead of a nice raven error, the SimpleVariableManager panics.

Raven Code To Reproduce
In core/src/string.rv:

impl Cast<str> for u64 {
    pub fn cast(self, number: u64) -> str {
        do {
            let digit = number % 10;
            let char_digit = '0' + digit;
            output = char_digit + output;
            number /= 10;
        } while(number != 0);
    }
}

Here, the variable output is undeclared.

Error Output or Unexpected Behavior

Unresolved variable output from SimpleVariableManager { variables: {"self": Reference(Struct(FinalizedStruct { generics: {}, fields: [],
 data: u64 })), "digit": Reference(Reference(Struct(FinalizedStruct { generics: {}, fields: [], data: u64 }))), "number": Reference(Stru
ct(FinalizedStruct { generics: {}, fields: [], data: u64 })), "char_digit": Reference(Struct(FinalizedStruct { generics: {}, fields: [],
 data: char }))} }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'test::test::test_magpie' panicked at language\runner\src\lib.rs:58:25:
task 157 panicked

As far as I can tell, it is mad that output is being used, but it isn't one of those fancy parsing/syntax errors that someone has made

Additional context
More testing should be done around this. I just found this error and haven't done much to reproduce it, as it is the second error I have found while just trying to do number->string casting. I'll try and update this with more info when/if I get around to it.