new pre-processing function

app.py

@@ -271,6 +271,8 @@ def process_family_sequence(protein_fasta):
 
 def process_single_sequence(protein_fasta): #, protein_file
     protein_sequence, error_msg = preprocess_protein_sequence(protein_fasta)
+    if error_msg:
+        return None, None, None, error_msg
 
     encoded_input = tokenizer([protein_sequence], padding=True, truncation=True, max_length=512, return_tensors="pt")
     input_ids = encoded_input["input_ids"]