Fix panic when fields of Failures are nil

[kofuk]

Reason and Arn which are referenced when RunTask failed can be nil.
This fixes panic raised when these fields are nil.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xc191d1]
goroutine 513097 [running]:
github.com/acidlemon/mirage-ecs/v2.(*ECS).launchTask(0xc0000a19b0, {0xf01750, 0xc0000bc230}, {0xc00182b500, 0x21}, {0xc001372660, 0x52}, 0xc00045dc20?)
	/stash/src/github.com/acidlemon/mirage-ecs/ecs.go:231 +0x971
github.com/acidlemon/mirage-ecs/v2.(*ECS).Launch.func1()
	/stash/src/github.com/acidlemon/mirage-ecs/ecs.go:256 +0x33
golang.org/x/sync/errgroup.(*Group).Go.func1()
	/stash/pkg/mod/golang.org/x/sync@v0.3.0/errgroup/errgroup.go:75 +0x56
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 416582

[kofuk]

@fujiwara
Could you take a look?

[fujiwara]

@kofuk I'm sorry it's too late. Thank you!

[kofuk]

Thank you very much!

[fujiwara]

v2.0.6 released! https://github.com/acidlemon/mirage-ecs/releases/tag/v2.0.6