name exceptions so we can ignore most of them

Now that we've had this in prod a while I've collected a few transient DNS
issues. Because webmentions are a nice-to-have, we can just drop the mention
if someone's DNS is flaking on us. We let BadIPsError hit logs because it
might be someone attempting to use this to enumerate our internal network
(tho it's most likely misconfiguration).

extras/sponge.rb

@@ -3,6 +3,10 @@ require "net/https"
 require "resolv"
 require "ipaddr"
 
+class BadIPsError < StandardError; end
+class DNSError < StandardError; end
+class NoIPsError < StandardError; end
+
 module Net
   class HTTP
     attr_accessor :address, :custom_conn_address, :skip_close
@@ -111,7 +115,7 @@ class Sponge
         ips = Resolv.getaddresses(uri.host)
 
         if !ips.any?
-          raise
+          raise NoIPsError
         end
 
         # reject ipv6 addresses
@@ -123,21 +127,21 @@ class Sponge
       end
     rescue Timeout::Error => e
       if retried
-        raise "couldn't resolve #{uri.host} (DNS timeout)"
+        raise DNSError.new("couldn't resolve #{uri.host} (DNS timeout)")
       else
         retried = true
         retry
       end
     rescue => e
-      raise "couldn't resolve #{uri.host} (#{e.inspect})"
+      raise DNSError.new("couldn't resolve #{uri.host} (#{e.inspect})")
     end
 
     if !ip
-      raise "couldn't resolve #{uri.host}"
+      raise DNSError.new("couldn't resolve #{uri.host}")
     end
 
     if BAD_NETS.select {|n| IPAddr.new(n).include?(ip) }.any?
-      raise "refusing to talk to IP #{ip}"
+      raise BadIPsError.new("refusing to talk to IP #{ip}")
     end
 
     host = Net::HTTP.new(ip.to_s, uri.port)

script/send_new_webmentions

@@ -78,9 +78,14 @@ if __FILE__ == $PROGRAM_NAME
 
     sp = Sponge.new
     sp.timeout = 10
-    response = sp.fetch(WEBrick::HTTPUtils.escape(s.url), :get, nil, nil, {
-      "User-agent" => "#{Rails.application.domain} webmention endpoint lookup",
-    }, 3)
+    begin
+      response = sp.fetch(WEBrick::HTTPUtils.escape(s.url), :get, nil, nil, {
+        "User-agent" => "#{Rails.application.domain} webmention endpoint lookup",
+      }, 3)
+    rescue NoIPsError, DNSError
+      # other people's DNS issues (usually transient); just skip the webmention
+      next
+    end
     next unless response
 
     wm_endpoint_raw = endpoint_from_headers(response['link']) ||