(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/ApiError.ts

@@ -0,0 +1,40 @@
+/**
+ * A tip for fixing an error.
+ */
+export interface ApiTip {
+  action: 'add-members' | 'upgrade' |'ask-for-help';
+  message: string;
+}
+
+/**
+ * Documentation of a limit relevant to an API error.
+ */
+export interface ApiLimit {
+  quantity: 'collaborators' | 'docs' | 'workspaces';  // what are we counting
+  subquantity?: string;    // a nuance to what we are counting
+  maximum: number;         // maximum allowed
+  value: number;           // current value of quantity for user
+  projectedValue: number;  // value of quantity expected if request had been allowed
+}
+
+/**
+ * Structured details about an API error.
+ */
+export interface ApiErrorDetails {
+  limit?: ApiLimit;
+
+  // If set, this is the more user-friendly message to show to the user than error.message.
+  userError?: string;
+
+  // If set, contains suggestions for fixing a problem.
+  tips?: ApiTip[];
+}
+
+/**
+ * An error with an http status code.
+ */
+export class ApiError extends Error {
+  constructor(message: string, public status: number, public details?: ApiErrorDetails) {
+    super(message);
+  }
+}