Added return code SSH_SERVER_FILE_NOT_FOUND

This error is returned by ssh_is_server_known when known_hosts
file does not exist and gives more action to the developer.

include/libssh/libssh.h

@@ -134,6 +134,7 @@ typedef int socket_t;
 #define SSH_SERVER_KNOWN_OK 1
 #define SSH_SERVER_KNOWN_CHANGED 2
 #define SSH_SERVER_FOUND_OTHER 3
+#define SSH_SERVER_FILE_NOT_FOUND 4
 
 #ifndef MD5_DIGEST_LEN
     #define MD5_DIGEST_LEN 16

libssh/keyfiles.c

@@ -1361,10 +1361,10 @@ int ssh_is_server_known(SSH_SESSION *session) {
   enter_function();
 
   if (ssh_options_default_known_hosts_file(session->options) < 0) {
-    ssh_set_error(session, SSH_FATAL,
+    ssh_set_error(session, SSH_REQUEST_DENIED,
         "Can't find a known_hosts file");
     leave_function();
-    return SSH_SERVER_ERROR;
+    return SSH_SERVER_FILE_NOT_FOUND;
   }
 
   if (session->options->host == NULL) {