Don't print a stacktrace on config errors

[julianbrost]

Throwing a stacktrace at a user isn't the nicest thing to do (looks more like the application did something wrong rather than the user did something wrong).

Exit code 2 seems to be what go does when you panic it, but invalid config is somewhat similar to invalid cli options (which also uses exit code 2 at the moment), so I kept that one.

Tests

Before

go run ./cmd/icingadb -c <(printf '{database: {type: invalid}}')
panic: invalid configuration: unknown database type "invalid", must be one of: "mysql", "pgsql"

goroutine 1 [running]:
github.com/icinga/icingadb/pkg/utils.Fatal(...)
	/home/jbrost/dev/icingadb/pkg/utils/utils.go:114
github.com/icinga/icingadb/internal/command.New()
	/home/jbrost/dev/icingadb/internal/command/command.go:41 +0x185
main.run()
	/home/jbrost/dev/icingadb/cmd/icingadb/main.go:40 +0x57
main.main()
	/home/jbrost/dev/icingadb/cmd/icingadb/main.go:36 +0x19
exit status 2

After

$ go run ./cmd/icingadb -c <(printf '{database: {type: invalid}}')
invalid configuration: unknown database type "invalid", must be one of: "mysql", "pgsql"
exit status 2