improve resilience

[svenwltr]

This improves the error handling. If a lister should fail aws-nuke will continue. It would look like this:

eu-west-1 - CloudFormationStack - 'acm-certificates' - filtered by config

=============

Listing with resources.ResourceLister failed:

runtime error: invalid memory address or nil pointer dereference

goroutine 27 [running]:
runtime/debug.Stack(0xe3fcc0, 0xe24f00, 0x17ffe60)
	/home/sven/Data/Applications/golang/src/runtime/debug/stack.go:24 +0xa7
github.com/rebuy-de/aws-nuke/cmd.safeLister.func1(0xc4200fff18)
	/home/sven/Data/Projects/src/github.com/rebuy-de/aws-nuke/cmd/scan.go:49 +0x75
panic(0xe24f00, 0x17ffe60)
	/home/sven/Data/Applications/golang/src/runtime/panic.go:491 +0x283
github.com/rebuy-de/aws-nuke/resources.(*EC2Nuke).ListInstances(0xc42000e110, 0xc4201ce300, 0xc4ad80, 0xc4200ffec0, 0xc4200ffec8, 0xc4200ffec0)
	/home/sven/Data/Projects/src/github.com/rebuy-de/aws-nuke/resources/ec2-instances.go:17 +0x31
github.com/rebuy-de/aws-nuke/resources.(*EC2Nuke).ListInstances-fm(0xc400000008, 0xffbb90, 0xc4200fff18, 0x0, 0x0)
	/home/sven/Data/Projects/src/github.com/rebuy-de/aws-nuke/resources/listers.go:55 +0x2a
github.com/rebuy-de/aws-nuke/cmd.safeLister(0xc4203f2540, 0x0, 0x0, 0x0, 0x0, 0x0)
	/home/sven/Data/Projects/src/github.com/rebuy-de/aws-nuke/cmd/scan.go:53 +0x7e
github.com/rebuy-de/aws-nuke/cmd.Scan.func1(0xc420001680, 0xc4201787e0)
	/home/sven/Data/Projects/src/github.com/rebuy-de/aws-nuke/cmd/scan.go:18 +0x25a
created by github.com/rebuy-de/aws-nuke/cmd.Scan
	/home/sven/Data/Projects/src/github.com/rebuy-de/aws-nuke/cmd/scan.go:14 +0x67


Please report this to https://github.com/rebuy-de/aws-nuke/issues/new.

=============

eu-west-1 - EC2KeyPair - 'sven.notebook' - filtered by config
eu-west-1 - EC2SpotFleetRequest - 'sfr-aadb907a-6586-439f-9865-8a489f24994b' - already cancelled
eu-west-1 - EC2SpotFleetRequest - 'sfr-b70670f1-8c23-43fe-84e1-f17ae495f7d2' - already cancelled

@rebuy-de/prp-aws-nuke Please review.