detect/content: Message for escaping backslash

So far, if the rule loaded had a backslash character ("\") in its
content field, the rule will fail to load but without giving a
descriptive error message. This patch tells the user to escape the
troubling character.

Before
```
9409] 7/6/2019 -- 16:12:22 - (detect-engine-loader.c:184) <Error> (DetectLoadSigFile) -- [ERRCODE: SC_ERR_INVALID_SIGNATURE(39)] - error parsing signature "alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"Backslash needs escaping msg"; flow:established,to_server; content:"MyBackslash\here"; sid:86; rev:1; )" from file /var/lib/suricata/rules/myrule.rules at line 1
```

After
```
[9409] 7/6/2019 -- 16:12:22 - (detect-content.c:155) <Error> (DetectContentDataParse) -- [ERRCODE: SC_ERR_INVALID_SIGNATURE(39)] - '\' has to be escaped
[9409] 7/6/2019 -- 16:12:22 - (detect-engine-loader.c:184) <Error> (DetectLoadSigFile) -- [ERRCODE: SC_ERR_INVALID_SIGNATURE(39)] - error parsing signature "alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"Backslash needs escaping msg"; flow:established,to_server; content:"MyBackslash\here"; sid:86; rev:1; )" from file /var/lib/suricata/rules/myrule.rules at line 1
```

Closes redmine ticket OISF#2626

src/detect-content.c

@@ -152,7 +152,7 @@ int DetectContentDataParse(const char *keyword, const char *contentstr,
                         str[x] = str[i];
                         x++;
                     } else {
-                        //SCLogDebug("Can't escape %c", str[i]);
+                        SCLogError(SC_ERR_INVALID_SIGNATURE, "'%c' has to be escaped", str[i-1]);
                         goto error;
                     }
                     escape = 0;