Unify dylib loading between proc macros and codegen backends

As bonus this makes the errors when failing to load a proc macro more
informative to match the backend loading errors. In addition it makes it
slightly easier to patch rustc to work on platforms that don't support
dynamic linking like wasm.

compiler/rustc_interface/src/lib.rs

@@ -1,5 +1,4 @@
 #![feature(decl_macro)]
-#![feature(error_iter)]
 #![feature(generic_nonzero)]
 #![feature(lazy_cell)]
 #![feature(let_chains)]