Added command to sanity check

Author: ODiogoSilva

structure_threader/sanity_checks/sanity.py

@@ -36,7 +36,8 @@ def log_error(self, msg, aux):
                       "Please correct the provided {f1}file and re-run"
                       " the plotting operation of structure_threader "
                       "with the following command:\n\n"
-                      "<command missing>".format(f1=aux, f2=msg))
+                      "structure_threader plot <opts>".format(
+            f1=aux, f2=msg))
         raise SystemExit
 
     def ind_mismatch(self, exp_array, kvals):