Fix error message when input validation check failed for select parameter

[mgmacias95]

Hello team,

When an invalid character was used with the select parameter, the following error was shown:

# curl -u foo:bar "localhost:55000/agents?pretty&select=os.platform"
{
   "error": "Undefined error.",
   "message": "Undefined error..  Field: select"
}

This is not correct since the error is not being controlled correctly. This PR fixes that, showing the following message:

# curl -u foo:bar "localhost:55000/agents?pretty&select=os.platform"
{
   "error": 619,
   "message": "Param not valid. Valid characters: a-z A-Z 0-9 space . , _.  Field: select"
}

Also, the character . has been added as valid to allow selecting fields such as os.platform and os.version:

# curl -u foo:bar "localhost:55000/agents?select=status,os.platform,os.version&pretty"
{
   "error": 0,
   "data": {
      "totalItems": 2,
      "items": [
         {
            "status": "Active",
            "os": {
               "platform": "centos",
               "version": "7"
            },
            "id": "000"
         },
         {
            "status": "Active",
            "os": {
               "platform": "centos",
               "version": "7"
            },
            "id": "001"
         }
      ]
   }
}

Best regards,
Marta