Report errors better when failing to open files for sub-parsers

src/test/compile-fail/mod_file_not_exist.rs

@@ -0,0 +1,5 @@
+mod not_a_real_file; //~ ERROR not_a_real_file.rs
+
+fn main() {
+    assert mod_file_aux::bar() == 10;
+}

src/test/compile-fail/mod_file_with_path_attr.rs

@@ -0,0 +1,6 @@
+#[path = "not_a_real_file.rs"]
+mod m; //~ ERROR not_a_real_file.rs
+
+fn main() {
+    assert m::foo() == 10;
+}