Convert some more macro panics to diagnostics (#611)

This should hopefully be the last of the manually written diagnostics!

crates/backend/src/lib.rs

@@ -17,8 +17,10 @@ extern crate wasm_bindgen_shared as shared;
 pub use codegen::TryToTokens;
 pub use error::Diagnostic;
 
+#[macro_use]
+mod error;
+
 pub mod ast;
 mod codegen;
 pub mod defined;
-mod error;
 pub mod util;