Better error messages

Right now the error messages look like this:

time="2019-12-11T19:36:35-05:00" level=fatal msg="docker-slim: failure" error="json: cannot unmarshal array into Go value of type docker.Image" stack="goroutine 1 [running]:\nruntime/debug.Stack(0x0, 0x0, 0xc0004c7e01)\n\truntime/debug/stack.go:24 +0x9d\ngithub.com/docker-slim/docker-slim/pkg/util/errutil.FailOn(0xb6be20, 0xc000218050)\n\tgithub.com/docker-slim/docker-slim@/pkg/util/errutil/errutil.go:14 +0x54\ngithub.com/docker-slim/docker-slim/internal/app/master/commands.OnBuild(0x9c4e01, 0xa92aae, 0x10, 0x0, 0x0, 0x0, 0x0, 0x0, 0xa80000, 0xc0001bef90, ...)\n\tgithub.com/docker-slim/docker-slim@/internal/app/master/commands/build.go:178 +0x91d\ngithub.com/docker-slim/docker-slim/internal/app/master.init.0.func6(0xc0001c8160, 0x0, 0xc0001ce2b0)\n\tgithub.com/docker-slim/docker-slim@/internal/app/master/cli.go:775 +0x1709\ngithub.com/urfave/cli.HandleAction(0x9b03a0, 0xab7478, 0xc0001c8160, 0xc0001c8160, 0x0)\n\tgithub.com/urfave/cli@v1.22.1/app.go:523 +0xbe\ngithub.com/urfave/cli.Command.Run(0xa8978f, 0x5, 0x0, 0x0, 0xc00017a580, 0x1, 0x1, 0xaab688, 0x3e, 0x0, ...)\n\tgithub.com/urfave/cli@v1.22.1/command.go:174 +0x51c\ngithub.com/urfave/cli.(*App).Run(0xc00018c000, 0xc000084150, 0x3, 0x3, 0x0, 0x0)\n\tgithub.com/urfave/cli@v1.22.1/app.go:276 +0x718\ngithub.com/docker-slim/docker-slim/internal/app/master.runCli()\n\tgithub.com/docker-slim/docker-slim@/internal/app/master/cli.go:1159 +0x55\ngithub.com/docker-slim/docker-slim/internal/app/master.Run()\n\tgithub.com/docker-slim/docker-slim@/internal/app/master/app.go:6 +0x25\nmain.main()\n\tgithub.com/docker-slim/docker-slim@/cmd/docker-slim/main.go:8 +0x20\n" version="linux|Transformer|1.26.1|2ec04e169b12a87c5286aa09ef44eac1cea2c7a1|2019-11-28_04:37:59PM"

I would rather they looked like this:

time="2019-12-11T19:36:35-05:00" level=fatal msg="docker-slim: failure" error="json: cannot unmarshal array into Go value of type docker.Image" stack="goroutine 1 [running]:
runtime/debug.Stack(0x0, 0x0, 0xc0004c7e01)
	runtime/debug/stack.go:24 +0x9d
github.com/docker-slim/docker-slim/pkg/util/errutil.FailOn(0xb6be20, 0xc000218050)
	github.com/docker-slim/docker-slim@/pkg/util/errutil/errutil.go:14 +0x54
github.com/docker-slim/docker-slim/internal/app/master/commands.OnBuild(0x9c4e01, 0xa92aae, 0x10, 0x0, 0x0, 0x0, 0x0, 0x0, 0xa80000, 0xc0001bef90, ...)
	github.com/docker-slim/docker-slim@/internal/app/master/commands/build.go:178 +0x91d
github.com/docker-slim/docker-slim/internal/app/master.init.0.func6(0xc0001c8160, 0x0, 0xc0001ce2b0)
	github.com/docker-slim/docker-slim@/internal/app/master/cli.go:775 +0x1709
github.com/urfave/cli.HandleAction(0x9b03a0, 0xab7478, 0xc0001c8160, 0xc0001c8160, 0x0)
	github.com/urfave/cli@v1.22.1/app.go:523 +0xbe
github.com/urfave/cli.Command.Run(0xa8978f, 0x5, 0x0, 0x0, 0xc00017a580, 0x1, 0x1, 0xaab688, 0x3e, 0x0, ...)
	github.com/urfave/cli@v1.22.1/command.go:174 +0x51c
github.com/urfave/cli.(*App).Run(0xc00018c000, 0xc000084150, 0x3, 0x3, 0x0, 0x0)
	github.com/urfave/cli@v1.22.1/app.go:276 +0x718
github.com/docker-slim/docker-slim/internal/app/master.runCli()
	github.com/docker-slim/docker-slim@/internal/app/master/cli.go:1159 +0x55
github.com/docker-slim/docker-slim/internal/app/master.Run()
	github.com/docker-slim/docker-slim@/internal/app/master/app.go:6 +0x25
main.main()
	github.com/docker-slim/docker-slim@/cmd/docker-slim/main.go:8 +0x20
" version="linux|Transformer|1.26.1|2ec04e169b12a87c5286aa09ef44eac1cea2c7a1|2019-11-28_04:37:59PM"

It looks like a string isn't being formatted somewhere maybe?

Start in pkg/util/errutil/errutil.go at errutil.FailOn, then trace the call from internal/app/master/commands/build.go. Reproduce the reported failure and compare the emitted stack trace with the issue's desired formatting; done means the error output preserves the intended line breaks and indentation.