gdb, btrace: Throw an error for empty recordings when replaying starts.

This makes record_btrace_start_replaying() more consistent, as it already
errors out e.g. on a recording with only gaps.

gdb/record-btrace.c

@@ -2017,7 +2017,7 @@ record_btrace_start_replaying (struct thread_info *tp)
 
   /* We can't start replaying without trace.  */
   if (btinfo->functions.empty ())
-    return NULL;
+    error (_("No trace."));
 
   /* GDB stores the current frame_id when stepping in order to detects steps
      into subroutines.