fix: add error messages for empty or failed exports (fixes #15)

src/export.js

@@ -51,16 +51,29 @@ exports.handler = async function (options) {
     const client = await getClient(options)
     const console = new Console(process.stderr)
 
+    if (options.entities.length === 0 && options.index.length === 0) {
+        process.stderr.write('Error: No entities specified. Use --entities to specify what to export.\n')
+        process.exit(1)
+    }
+
     console.time('Export')
     return client
         .export(options.format, [...options.entities, ...options.index], options.outputFile)
         .then(entities => {
             console.timeEnd('Export')
 
-            for (const { status, reason } of entities) {
-                if (status === 'rejected') {
-                    console.error(reason)
-                }
+            const rejected = entities.filter(e => e.status === 'rejected')
+            const fulfilled = entities.filter(e => e.status === 'fulfilled')
+
+            for (const { reason } of rejected) {
+                process.stderr.write(`Error: ${reason.message}\n`)
+            }
+
+            if (rejected.length > 0 && fulfilled.length === 0) {
+                process.stderr.write(`\nAll exports failed. Please check your format and entity options.\n`)
+                process.stderr.write(`Note: Some formats like 'atf' may not be available for direct export.\n`)
+                process.stderr.write(`Try using a different format such as 'ndjson', 'csv', or 'ntriples'.\n`)
+                process.exit(1)
             }
         })
 }