Fix linking option for library dataset uploads

Broken in commit a45cbfe .

Found by BioBlend tests:
https://travis-ci.org/galaxyproject/bioblend/jobs/356761960

tools/data_source/upload.py

@@ -68,9 +68,9 @@ def add_file(dataset, registry, output_path):
     converted_path = None
     stdout = None
     link_data_only_str = dataset.get('link_data_only', 'copy_files')
-    if link_data_only_str not in ['link_data_only', 'copy_files']:
-        raise UploadProblemException("Invalid setting for option link_data_only - upload request misconfigured.")
-    link_data_only = link_data_only_str == 'link_data_only'
+    if link_data_only_str not in ['link_to_files', 'copy_files']:
+        raise UploadProblemException("Invalid setting '%s' for option link_data_only - upload request misconfigured" % link_data_only_str)
+    link_data_only = link_data_only_str == 'link_to_files'
 
     # run_as_real_user is estimated from galaxy config (external chmod indicated of inputs executed)
     # If this is True we always purge supplied upload inputs so they are cleaned up and we reuse their