Format sources into the error message when loading codegen backends

cc https://github.com/rust-lang/rustc_codegen_cranelift/issues/1447

compiler/rustc_interface/src/lib.rs

@@ -1,9 +1,10 @@
 #![feature(box_patterns)]
 #![feature(decl_macro)]
+#![feature(error_iter)]
 #![feature(internal_output_capture)]
-#![feature(thread_spawn_unchecked)]
 #![feature(lazy_cell)]
 #![feature(let_chains)]
+#![feature(thread_spawn_unchecked)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]