scanner: do not emit layout_empty during error recovery

This token is only valid after a block, and it is impossible to see errors
here (since this node corresponds to literally nothing).

Don't emit this during error recovery to prevent strange interpretations.

corpus/errors.txt

@@ -0,0 +1,25 @@
+================================================================================
+Error within variant object declaration
+================================================================================
+
+type
+  E = object
+    case foo
+    of bar
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (type_section
+    (type_declaration
+      (type_symbol_declaration
+        (identifier))
+      (object_declaration
+        (field_declaration_list
+          (variant_declaration
+            (variant_discriminator_declaration
+              (symbol_declaration_list
+                (symbol_declaration
+                  (identifier)))))
+          (ERROR
+            (identifier)))))))

src/scanner.cc

@@ -814,7 +814,11 @@ bool lex_indent(Context& ctx)
     return ctx.finish(TokenType::LayoutStart);
   }
 
-  if (ctx.valid(TokenType::LayoutEmpty) &&
+  // LayoutEmpty has to be explicitly requested, and errors generally
+  // don't happen within the contexts it would be requested.
+  //
+  // Don't emit them on errors to prevent mangling error recovery.
+  if (!ctx.error() && ctx.valid(TokenType::LayoutEmpty) &&
       ctx.state().test_flag(Flag::AfterNewline) &&
       (current_layout >= line_indent || ctx.eof())) {
     ctx.mark_end();