Improve PHP error shown for missing config file

Improve the current 403 message for when config file is missing or empty from
````{"error":{
    "code":403,
    "details":["This proxy does not support empty parameters."],
    "message":"This proxy does not support empty parameters."
}}````
to 
````{"error":{
    "code":403,"
    details":["Proxy error: problem reading proxy configuration file."],
    "message":"Proxy error: problem reading proxy configuration file."
}}````