dnsdist: allow known exception types to be converted to string

[chbruyand]

Short description

This tries to improve Lua error reporting, see #6535. Exceptions thrown in a Lua subroutine can be printed:

x = newNMG()
res, err = pcall(function() x:addMask('banana') end)
if not res then
   print("An error has been thrown: ", err)
end

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[chbruyand]

For the record, we could also catch some specific exception and convert them to lua_error directly in the Lua wrapper : https://gist.github.com/chbruyand/586820716b128cf680d12599fd46adc7

This allows to get more details about the error

res, err = pcall(function() throwSomeException() end)
if not res then
   print (err)
end

will output

[string "chunk"]:1: Caught exception: Something bad happened

But it would also require to catch PDNSExceptionss as they diverge from std::exceptions, which would not be so clean.

[zeha]

But it would also require to catch PDNSExceptionss as they diverge from std::exceptions, which would not be so clean.

Maybe its time to ditch PDNSException or make it a std subclass.

[ahupowerdns]

looks useful!