Support configurability of the NGX_RESOLVE_ errors to use fallback for

[nicholaschiasson]

Currently, we have hardcoded the cases of NGX_RESOLVE_FORMERR and NGX_RESOLVE_NXDOMAIN as errors for which we always use the fallback. It would be more interesting to allow the configuration to select which resolve errors should always use the fallback. This could possibly be an enhancement of the strict attribute.

See here for list of errors: http://lxr.nginx.org/source/xref/nginx/src/core/ngx_resolver.h#27

#define NGX_RESOLVE_FORMERR   1
#define NGX_RESOLVE_SERVFAIL  2
#define NGX_RESOLVE_NXDOMAIN  3
#define NGX_RESOLVE_NOTIMP    4
#define NGX_RESOLVE_REFUSED   5
#define NGX_RESOLVE_TIMEDOUT  NGX_ETIMEDOUT