expand: Introduce enum for module loading errors and make module loading speculative

src/test/ui/directory_ownership/macro-expanded-mod.rs

@@ -2,7 +2,7 @@
 
 macro_rules! mod_decl {
     ($i:ident) => {
-        mod $i; //~ ERROR Cannot declare a non-inline module inside a block
+        mod $i; //~ ERROR cannot declare a non-inline module inside a block
     };
 }
 

src/test/ui/directory_ownership/macro-expanded-mod.stderr

@@ -1,4 +1,4 @@
-error: Cannot declare a non-inline module inside a block unless it has a path attribute
+error: cannot declare a non-inline module inside a block unless it has a path attribute
   --> $DIR/macro-expanded-mod.rs:5:9
    |
 LL |         mod $i;

src/test/ui/directory_ownership/non-inline-mod-restriction.rs

@@ -1,5 +1,5 @@
 // Test that non-inline modules are not allowed inside blocks.
 
 fn main() {
-    mod foo; //~ ERROR Cannot declare a non-inline module inside a block
+    mod foo; //~ ERROR cannot declare a non-inline module inside a block
 }

src/test/ui/directory_ownership/non-inline-mod-restriction.stderr

@@ -1,4 +1,4 @@
-error: Cannot declare a non-inline module inside a block unless it has a path attribute
+error: cannot declare a non-inline module inside a block unless it has a path attribute
   --> $DIR/non-inline-mod-restriction.rs:4:5
    |
 LL |     mod foo;