add "Fail" transport, that immediately fails delivery

allows configs that prevent outgoing deliveries (globally, per domain,
or per account) from/to certain domains.

for issue #347

webadmin/api.json

@@ -6901,6 +6901,14 @@
 						"nullable",
 						"TransportDirect"
 					]
+				},
+				{
+					"Name": "Fail",
+					"Docs": "",
+					"Typewords": [
+						"nullable",
+						"TransportFail"
+					]
 				}
 			]
 		},
@@ -7022,6 +7030,40 @@
 				}
 			]
 		},
+		{
+			"Name": "TransportFail",
+			"Docs": "TransportFail is a transport that fails all delivery attempts.",
+			"Fields": [
+				{
+					"Name": "SMTPCode",
+					"Docs": "",
+					"Typewords": [
+						"int32"
+					]
+				},
+				{
+					"Name": "SMTPMessage",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "Code",
+					"Docs": "Effective values to use, set when parsing.",
+					"Typewords": [
+						"int32"
+					]
+				},
+				{
+					"Name": "Message",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				}
+			]
+		},
 		{
 			"Name": "EvaluationStat",
 			"Docs": "EvaluationStat summarizes stored evaluations, for inclusion in an upcoming\naggregate report, for a domain.",