Move RelayError into its own file, add helper methods

src/RelayError.ts

@@ -0,0 +1,24 @@
+import { NostrRelayOK } from '@nostrify/nostrify';
+
+export type RelayErrorPrefix = 'duplicate' | 'pow' | 'blocked' | 'rate-limited' | 'invalid' | 'error';
+
+/** NIP-01 command line result. */
+export class RelayError extends Error {
+  constructor(prefix: RelayErrorPrefix, message: string) {
+    super(`${prefix}: ${message}`);
+  }
+
+  /** Construct a RelayError from the reason message. */
+  static fromReason(reason: string): RelayError {
+    const [prefix, ...rest] = reason.split(': ');
+    return new RelayError(prefix as RelayErrorPrefix, rest.join(': '));
+  }
+
+  /** Throw a new RelayError if the OK message is false. */
+  static assert(msg: NostrRelayOK): void {
+    const [_, _eventId, ok, reason] = msg;
+    if (!ok) {
+      throw RelayError.fromReason(reason);
+    }
+  }
+}