Panic during startup when starting proxy with an unsupported resolver (libunbound)

[buffrr]

unbound doesn't support DoH so a common error is:

$ letsdane -r https://1.1.1.1

This crashes with

panic: runtime error: invalid memory address or nil pointer dereference

Since it tries to clean up the unbound instance even though it has not been created yet. It should show the actual error given by libunbound instead.