If your request is correct and API sends a valid response to you, HTTP status code is 200. All other status codes except 200 means there is an error and response is similar following JSON string.

{
"error": {
"type": "<error type>",
"message": "<error message>"
}
}

Examples
When username or password is not right while login
{
"error": {
"type": "UNAUTHORIZED",
"message": "Your username or password is invalid."
}
}

When an item# is not found
{
"error": {
"type": "NOT_FOUND",
"message": "The Item# is not found, in the Catalog. Please add to catalog and retry"
}
}

When a location is not found
{
"error": {
"type": "NOT_FOUND",
"message": "Your location is not found"
}
}