New spago args (--json-errors)

[wclr]

This is just in case. New spago assumes passing --json-errors without --purs-args: spago build --json-errors, currently by default LS uses spago build --purs-args --json-errors build command wich gives an error.

So one can use buildCommand option to override the default build command.

[nwolverson]

Yes, this for sure should become the default eventually, I guess new spago is still officially alpha, not sure what the usage % now splits at. That's assuming it isn't supported in old-spago without --purs-args