feat: add better error messages / linked error with cause for new rpc v2 error cases

Author: 1egoman

src/room/rpc/client/RpcClientManager.ts

@@ -197,7 +197,7 @@ export default class RpcClientManager extends (EventEmitter as new () => TypedEm
       this.log.warn(`Error reading RPC response payload: ${e}`);
       this.handleIncomingRpcResponseFailure(
         associatedRequestId,
-        RpcError.builtIn('APPLICATION_ERROR'),
+        RpcError.builtIn('APPLICATION_ERROR', 'Error reading RPC response payload', { cause: e }),
       );
       return;
     }

src/room/rpc/server/RpcServerManager.ts

@@ -105,6 +105,7 @@ export default class RpcServerManager extends (EventEmitter as new () => TypedEm
         responseError = RpcError.builtIn(
           'APPLICATION_ERROR',
           `Uncaught error: ${(error as Error)?.message ?? error}`,
+          { cause: error },
         );
       }
 
@@ -141,7 +142,7 @@ export default class RpcServerManager extends (EventEmitter as new () => TypedEm
         callerIdentity,
         requestId,
         null,
-        RpcError.builtIn('APPLICATION_ERROR'),
+        RpcError.builtIn('APPLICATION_ERROR', 'RPC data stream malformed'),
       );
       return;
     }
@@ -167,7 +168,7 @@ export default class RpcServerManager extends (EventEmitter as new () => TypedEm
         callerIdentity,
         requestId,
         null,
-        RpcError.builtIn('APPLICATION_ERROR'),
+        RpcError.builtIn('APPLICATION_ERROR', 'Error reading RPC request payload', { cause: e }),
       );
       return;
     }

src/room/rpc/utils.ts

@@ -60,18 +60,25 @@ export class RpcError extends Error {
 
   data?: string;
 
+  // More info: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/cause
+  cause?: unknown;
+
   /**
    * Creates an error object with the given code and message, plus an optional data payload.
    *
    * If thrown in an RPC method handler, the error will be sent back to the caller.
    *
    * Error codes 1001-1999 are reserved for built-in errors (see RpcError.ErrorCode for their meanings).
    */
-  constructor(code: number, message: string, data?: string) {
+  constructor(code: number, message: string, data?: string, options?: { cause?: unknown }) {
     super(message);
     this.code = code;
     this.message = truncateBytes(message, RpcError.MAX_MESSAGE_BYTES);
     this.data = data ? truncateBytes(data, RpcError.MAX_DATA_BYTES) : undefined;
+
+    if (typeof options?.cause !== 'undefined') {
+      this.cause = options?.cause;
+    }
   }
 
   /**