dnsdist: in error messages show which parameter could not be converted

[zeha]

• Program: dnsdist
• Issue type: Feature request

Short description

I was holding setPoolServerPolicy wrong, and the error message didn't help me.

Usecase

Making runtime config changes before coffee.

Description

> setPoolServerPolicy("poolname", leastOutstanding)
Error: Unable to convert parameter from string to St10shared_ptrI12ServerPolicyE
stack traceback:
	[C]: at 0xaaaacfa7ab04

> leastOutstanding
Command returned an object we can't print: Trying to cast a lua variable from "userdata" to "N5boost8optionalINS_7variantINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEJSt10shared_ptrI15DownstreamStateEP11ClientStateSt13unordered_mapIS7_dSt4hashIS7_ESt8equal_toIS7_ESaISt4pairIKS7_dEEEEEEEE"

> setPoolServerPolicy("poolname", "leastOutstanding")
Error: Unable to convert parameter from string to St10shared_ptrI12ServerPolicyE
stack traceback:
	[C]: at 0xaaaacfa7ab04>

After @pieterlexis helped me:

> setPoolServerPolicy(leastOutstanding, "nextdns")

So what I think would have helped me:

• Instead of "Error: Unable to convert parameter from string to St10shared_ptrI12ServerPolicyE" print something like "Error: Unable to convert parameter 2 from string to St10shared_ptrI12ServerPolicyE" - show the parameter position that failed
• make builtin objects like leastOutstanding printable. Just "Builtin ServerPolicy leastOutstanding" would already be super useful.

Additionally it would be nice if functions named set<ThingA><ThingB> would take parameters in order <ThingA> <ThingB> and not the other way round.