Add internal error handling to VM, plus function arity

* VM is able to handle basic runtime errors although there is no way to
  catch this in executing code currently
* If a function is called with the wrong number of arguments (arity) it
  will invoke a runtime error.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/error.rs

@@ -1,9 +1,24 @@
+use crate::obj::prelude::*;
 use snafu::Snafu;
 
 #[derive(Debug, Snafu)]
 pub enum Error {
-    #[snafu(display("attempted to pop an empty stack"))]
-    EmptyStack,
+    #[snafu(display("missing attribute: {}", global_sym_lookup(*attr).unwrap()))]
+    MissingAttr {
+        attr: Sym,
+    },
+
+    #[snafu(display("{}", error))]
+    ValueError {
+        error: String,
+        value: ObjRef,
+    },
+
+    #[snafu(display("incorrect function arity; expected {} but got {} instead", expected, got))]
+    ArityError {
+        expected: usize,
+        got: usize,
+    },
 }
 
 pub type Result<T, E = Error> = std::result::Result<T, E>;