Better error reporting when a dataset doesn't exist.

megatron/data/indexed_dataset.py

@@ -43,7 +43,8 @@ def infer_dataset_impl(path):
             else:
                 return None
     else:
-        print(f"Dataset path does not exist: {path}")
+        print(f"Dataset does not exist: {path}")
+        print("Path should be a basename that both .idx and .bin can be appended to get full filenames.")
         return None
 
 
@@ -55,6 +56,10 @@ def make_builder(out_file, impl, vocab_size=None):
 
 
 def make_dataset(path, impl, skip_warmup=False):
+    if not IndexedDataset.exists(path):
+        print(f"Dataset does not exist: {path}")
+        print("Path should be a basename that both .idx and .bin can be appended to get full filenames.")
+        return None
     if impl == 'infer':
         impl = infer_dataset_impl(path)
     if impl == 'lazy' and IndexedDataset.exists(path):