Add EmptyStackError class

This error will be thrown any time an item is supposed to be removed
from the stack, but can't due to it having no items.

src/errors.ts

@@ -22,4 +22,10 @@ export class FailedToParseCodeBoxError extends StarfishError {
     constructor(badCodeBoxString: string|string[]|string[][]) {
         super("Failed to parse CodeBox input.\n"+util.inspect(badCodeBoxString));
     }
+}
+
+export class EmptyStackError extends StarfishError {
+    constructor() {
+        super("Unable to perform this operation on an empty stack");
+    }
 }