feat: Add file existence check before processing input filename

gtm

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 import curses
+import os
 import subprocess
 import sys
 
@@ -239,7 +240,14 @@ if __name__ == "__main__":
         elif sys.argv[1] == "-h" or sys.argv[1] == "--help":
             show_help()
             sys.exit(0)
+        
         filename = sys.argv[1]
+        
+        # Check if the file exists
+        if not os.path.isfile(filename):
+            print(f"Error: File '{filename}' does not exist")
+            sys.exit(1)
+            
         curses.wrapper(main, filename)
     else:
         show_help()