BUG/MINOR: vars: make parse_store() return error on var_set() failure

BUG/MINOR: vars: make parse_store() return error on var_set() failure

In 2.5, variables in the scope "proc" were pre-created with commit
df8eeb1619 ("MEDIUM: vars: pre-create parsed SCOPE_PROC variables as
permanent ones"). However one test on var_set() was copy-pasted from
vars_check_args() into parse_store(), and the former returns 0 on
error while for the latter it's a success. This means that some errors
on variables of scope "proc" (typically alloc failure) can be missed
at boot time, probably either making that variable invisible or causing
a crash during boot.

Let's return ACT_RET_PRS_ERR instead. This can be backported.