(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/common/ErrorWithCode.ts

@@ -0,0 +1,23 @@
+import {OpenDocMode} from 'app/common/DocListAPI';
+
+interface ErrorDetails {
+  status?: number;
+  accessMode?: OpenDocMode;
+}
+
+/**
+ *
+ * An error with a human-readable message and a machine-readable code.
+ * Makes it easier to change the human-readable message without breaking
+ * error handlers.
+ *
+ */
+export class ErrorWithCode extends Error {
+  public accessMode?: OpenDocMode;
+  public status?: number;
+  constructor(public code: string, message: string, details: ErrorDetails = {}) {
+    super(message);
+    this.status = details.status;
+    this.accessMode = details.accessMode;
+  }
+}