Replace SERVER_ERROR with SERVFAIL

If only because SERVFAIL can be looked up on Google

ChangeLog

@@ -17,9 +17,9 @@ service if they temporarily can't be refreshed.
 issue when initialization took more than 30 seconds ("The service did not
 respond to the start or control request in a timely fashion"). Fantastic
 work by Alison Winters, thanks!
- - A new error code, NETWORK_ERROR, has been introduced in order to make
-the distinction between network errors and servers responding with a SERVFAIL
-error code.
+ - The `SERVER_ERROR` error code has been split into two new error code:
+`NETWORK_ERROR` (self-explanatory) and `SERVFAIL` (a response was returned,
+but it includes a `SERVFAIL` error code).
 
 * Version 2.0.31
  - This version fixes two regressions introduced in version 2.0.29:

dnscrypt-proxy/plugins.go

@@ -42,7 +42,7 @@ const (
 	PluginsReturnCodeParseError
 	PluginsReturnCodeNXDomain
 	PluginsReturnCodeResponseError
-	PluginsReturnCodeServerError
+	PluginsReturnCodeServFail
 	PluginsReturnCodeNetworkError
 	PluginsReturnCodeCloak
 	PluginsReturnCodeServerTimeout
@@ -57,7 +57,7 @@ var PluginsReturnCodeToString = map[PluginsReturnCode]string{
 	PluginsReturnCodeParseError:    "PARSE_ERROR",
 	PluginsReturnCodeNXDomain:      "NXDOMAIN",
 	PluginsReturnCodeResponseError: "RESPONSE_ERROR",
-	PluginsReturnCodeServerError:   "SERVER_ERROR",
+	PluginsReturnCodeServFail:      "SERVFAIL",
 	PluginsReturnCodeNetworkError:  "NETWORK_ERROR",
 	PluginsReturnCodeCloak:         "CLOAK",
 	PluginsReturnCodeServerTimeout: "SERVER_TIMEOUT",
@@ -285,7 +285,7 @@ func (pluginsState *PluginsState) ApplyResponsePlugins(pluginsGlobals *PluginsGl
 	case dns.RcodeNameError:
 		pluginsState.returnCode = PluginsReturnCodeNXDomain
 	case dns.RcodeServerFailure:
-		pluginsState.returnCode = PluginsReturnCodeServerError
+		pluginsState.returnCode = PluginsReturnCodeServFail
 	default:
 		pluginsState.returnCode = PluginsReturnCodeResponseError
 	}