Don't require remote server list fields in config

* Previously, client would exit with an error when
  remote server list config fields were blank.
* Helps automated testing cases to allow blank fields
* Check for blank fields still happens, but only
  when remote server list fetch runs, and result
  is an error message but not client exit

psiphon/config.go

@@ -111,14 +111,6 @@ func LoadConfig(configJson []byte) (*Config, error) {
 		return nil, ContextError(
 			errors.New("sponsor ID is missing from the configuration file"))
 	}
-	if config.RemoteServerListUrl == "" {
-		return nil, ContextError(
-			errors.New("remote server list URL is missing from the configuration file"))
-	}
-	if config.RemoteServerListSignaturePublicKey == "" {
-		return nil, ContextError(
-			errors.New("remote server list signature public key is missing from the configuration file"))
-	}
 
 	if config.DataStoreDirectory == "" {
 		config.DataStoreDirectory, err = os.Getwd()

psiphon/remoteServerList.go

@@ -20,6 +20,7 @@
 package psiphon
 
 import (
+	"errors"
 	"io/ioutil"
 	"net/http"
 )
@@ -31,6 +32,13 @@ import (
 func FetchRemoteServerList(config *Config, dialConfig *DialConfig) (err error) {
 	NoticeInfo("fetching remote server list")
 
+	if config.RemoteServerListUrl == "" {
+		return ContextError(errors.New("remote server list URL is blank"))
+	}
+	if config.RemoteServerListSignaturePublicKey == "" {
+		return ContextError(errors.New("remote server list signature public key blank"))
+	}
+
 	transport := &http.Transport{
 		Dial: NewTCPDialer(dialConfig),
 	}