degesch: show an error message on log write failure

Running out of space and I/O errors seem like the most likely causes.

degesch.c

@@ -12738,7 +12738,8 @@ on_display_full_log (int count, int key, void *user_data)
 	}
 
 	if (ctx->current_buffer->log_file)
-		fflush (ctx->current_buffer->log_file);
+		// The regular flush will log any error eventually
+		(void) fflush (ctx->current_buffer->log_file);
 
 	set_cloexec (fileno (full_log));
 	launch_backlog_helper (ctx, fileno (full_log));
@@ -13631,9 +13632,23 @@ rearm_flush_timer (struct app_context *ctx)
 static void
 on_flush_timer (struct app_context *ctx)
 {
-	// TODO: maybe also garbage collect all plugins?
 	// I guess we don't need to do anything more complicated
 	fflush (NULL);
+
+	// It would be a bit problematic to handle it properly, so do this at least
+	LIST_FOR_EACH (struct buffer, buffer, ctx->buffers)
+	{
+		if (!buffer->log_file || !ferror (buffer->log_file))
+			continue;
+
+		// Might be a transient error such as running out of disk space,
+		// keep notifying of the problem until it disappears
+		clearerr (buffer->log_file);
+		log_global (ctx, BUFFER_LINE_ERROR | BUFFER_LINE_SKIP_FILE,
+			"Log write failure detected for #s", buffer->name);
+	}
+
+	// TODO: maybe also garbage collect all plugins?
 	rearm_flush_timer (ctx);
 }