mca_base_var: use ERR_NOT_FOUND when var is inactive

After discussion with Nathan, change the ERR_VALUE_OUT_OF_BOUNDS to be
ERR_NOT_FOUND, for two reasons:

1. It's consistent with other uses of ERR_NOT_FOUND in the code.
1. In this case, we could have just looked up a variable that is
   basically a "hole" -- e.g., var indexes 0, 1, 2 are valid, and 4,
   5, and 5 are valid, but 3 is invalid (e.g., 3 was de-registered --
   remember that MPI_T explicitly does not allow re-using indexes).
   So returning ERR_OUT_OF_BOUNDS seems weird -- returning
   ERR_NOT_FOUND seems a bit more natural.

cmr=v1.8.2:ticket=trac:4587

This commit was SVN r32158.

The following Trac tickets were found above:
  Ticket 4587 --> https://svn.open-mpi.org/trac/ompi/ticket/4587

opal/mca/base/mca_base_var.c

@@ -424,7 +424,7 @@ int mca_base_var_get_value (int vari, const void *value,
     }
 
     if (!VAR_IS_VALID(var[0])) {
-        return OPAL_ERR_VALUE_OUT_OF_BOUNDS;
+        return OPAL_ERR_NOT_FOUND;
     }
 
     if (NULL != value) {
@@ -843,7 +843,7 @@ int mca_base_var_get (int vari, const mca_base_var_t **var)
     }
 
     if (!VAR_IS_VALID(*(var[0]))) {
-        return OPAL_ERR_VALUE_OUT_OF_BOUNDS;
+        return OPAL_ERR_NOT_FOUND;
     }
 
     return OPAL_SUCCESS;