List supported currency pairs

{
"data": {
"items": [
  {
    "currency_pair": "USD_NGN",
    "from_currency": "USD",
    "from_currency_name": "US Dollar",
    "to_currency": "NGN",
    "to_currency_name": "Nigerian Naira"
  }
]
},
"error": null
}

{
"data": null,
"error": "internal server error"
}