add message.h and message.c, mscp_set|get_error()

Instead of pr_err(), libmscp uses mscp_set_error() and
applications use mscp_get_errror() to get error message.

src/message.c

@@ -0,0 +1,25 @@
+#include <stdio.h>
+#include <stdarg.h>
+#include <string.h>
+#include <limits.h>
+
+#include <message.h>
+
+#define MSCP_ERRMSG_SIZE	(PATH_MAX * 2)
+
+static char errmsg[MSCP_ERRMSG_SIZE];
+
+void _mscp_set_error(const char *fmt, ...)
+{
+	va_list va;
+
+	memset(errmsg, 0, sizeof(errmsg));
+	va_start(va, fmt);
+	vsnprintf(errmsg, sizeof(errmsg) - 1, fmt, va);
+	va_end(va);
+}
+
+const char *mscp_get_error()
+{
+	return errmsg;
+}