Nicer panic when trying to insert instruction before calling switch_to_block

lib/frontend/src/frontend.rs

@@ -367,7 +367,8 @@ impl<'a> FunctionBuilder<'a> {
     /// Returns an object with the [`InstBuilder`](../codegen/ir/builder/trait.InstBuilder.html)
     /// trait that allows to conveniently append an instruction to the current `Ebb` being built.
     pub fn ins<'short>(&'short mut self) -> FuncInstBuilder<'short, 'a> {
-        let ebb = self.position.ebb.unwrap();
+        let ebb = self.position.ebb
+            .expect("Please call switch_to_block before inserting instructions");
         FuncInstBuilder::new(self, ebb)
     }