* 'error' function fo backend

src/backend.c

@@ -30,6 +30,8 @@ f2b_backend_create(f2b_config_section_t *config, const char *id) {
     goto cleanup;
   if ((*(void **) (&backend->ready)   = dlsym(backend->h, "ready"))   == NULL)
     goto cleanup;
+  if ((*(void **) (&backend->error)   = dlsym(backend->h, "error"))   == NULL)
+    goto cleanup;
   if ((*(void **) (&backend->start)   = dlsym(backend->h, "start"))   == NULL)
     goto cleanup;
   if ((*(void **) (&backend->stop)    = dlsym(backend->h, "stop"))    == NULL)

src/backend.h

@@ -10,6 +10,7 @@ typedef struct f2b_backend_t {
   void *(*create)  (const char *id);
   bool  (*config)  (void *cfg, const char *key, const char *value);
   bool  (*ready)   (void *cfg);
+  char  (*error)   (void *cfg);
   bool  (*start)   (void *cfg);
   bool  (*stop)    (void *cfg);
   bool  (*ping)    (void *cfg);

src/backends/backend.h

@@ -9,6 +9,7 @@ typedef struct _config cfg_t;
 extern cfg_t *create(const char *id);
 extern bool   config(cfg_t *c, const char *key, const char *value);
 extern bool    ready(cfg_t *c);
+extern char   *error(cfg_t *c);
 extern bool    start(cfg_t *c);
 extern bool     stop(cfg_t *c);
 extern bool     ping(cfg_t *c);