Refactor diagnostics in handle_errors function

compiler/rustc_error_messages/locales/en-US/attr.ftl

@@ -0,0 +1,29 @@
+attr_multiple_item =
+    multiple '{$item}' items
+
+attr_unknown_meta_item =
+    unknown meta item '{$item}'
+    .label = expected one of {$expected}
+
+attr_missing_since =
+    missing 'since'
+
+attr_non_ident_feature =
+    'feature' is not an identifier
+
+attr_missing_feature =
+    missing 'feature'
+
+attr_multiple_stability_levels =
+    multiple stability levels
+
+attr_unsupported_literal_generic =
+    unsupported literal
+attr_unsupported_literal_cfg_string =
+    literal in `cfg` predicate value must be a string
+attr_unsupported_literal_deprecated_string =
+    literal in `deprecated` value must be a string
+attr_unsupported_literal_deprecated_kv_pair =
+    item in `deprecated` must be a key/value pair
+attr_unsupported_literal_suggestion =
+    consider removing the prefix

compiler/rustc_error_messages/src/lib.rs

@@ -33,6 +33,7 @@ pub use unic_langid::{langid, LanguageIdentifier};
 // Generates `DEFAULT_LOCALE_RESOURCES` static and `fluent_generated` module.
 fluent_messages! {
     ast_passes => "../locales/en-US/ast_passes.ftl",
+    attr => "../locales/en-US/attr.ftl",
     borrowck => "../locales/en-US/borrowck.ftl",
     builtin_macros => "../locales/en-US/builtin_macros.ftl",
     const_eval => "../locales/en-US/const_eval.ftl",