DEBUG ERROR implemented.

The new "error" subcommand of the DEBUG command can reply with an user
selected error, specified as its sole argument:

    DEBUG ERROR "LOADING please wait..."

The error is generated just prefixing the command argument with a "-"
character, and replacing newlines with spaces (since error replies can't
include newlines).

The goal of the command is to help in Client libraries unit tests by
making simple to simulate a command call triggering a given error.

src/debug.c

@@ -363,6 +363,13 @@ void debugCommand(redisClient *c) {
     {
         server.active_expire_enabled = atoi(c->argv[2]->ptr);
         addReply(c,shared.ok);
+    } else if (!strcasecmp(c->argv[1]->ptr,"error") && c->argc == 3) {
+        sds errstr = sdsnewlen("-",1);
+
+        errstr = sdscatsds(errstr,c->argv[2]->ptr);
+        errstr = sdsmapchars(errstr,"\n\r","  ",2); /* no newlines in errors. */
+        errstr = sdscatlen(errstr,"\r\n",2);
+        addReplySds(c,errstr);
     } else {
         addReplyErrorFormat(c, "Unknown DEBUG subcommand or wrong number of arguments for '%s'",
             (char*)c->argv[1]->ptr);