[BridgeAbstract] Add returnClientError and returnServerError

BridgeAbstract.md

@@ -189,10 +189,14 @@ All bridges extend from `BridgeAbstract` and therefore are able to make use of f
 Following functions should be used for good practice and will support you with your bridge :
 
 * [`returnError`](#the-returnerror-function)
+* [`returnClientError`](#the-returnclienterror-function)
+* [`returnServerError`](#the-returnservererror-function)
 * [`file_get_html`](#the-file_get_html-function)
 
 ## The `returnError` function
 
+**Notice:** Whenever possible make use of [`returnClientError`](#the-returnclienterror-function) or [`returnServerError`](#the-returnservererror-function)
+
 This function aborts execution of the current bridge and returns the given error message with the provided error number :
 
 ```PHP
@@ -201,6 +205,26 @@ $this->returnError('Your error message', 404)
 
 Check the [list of error codes](https://en.wikipedia.org/wiki/List_of_HTTP_status_codes) for applicable error numbers.
 
+## The `returnClientError` function
+
+This function aborts execution of the current bridge and returns the given error message with error code **400**:
+
+```PHP
+$this->returnClientError('Your error message')
+```
+
+Use this function when the user provided invalid parameter or a required parameter is missing.
+
+## The `returnServerError` function
+
+This function aborts execution of the current bridge and returns the given error message with error code **500**:
+
+```PHP
+$this->returnServerError('Your error message')
+```
+
+Use this function when a problem occurs that has nothing to do with the parameters provided by the user. (like: Host service gone missing, empty data received, etc...)
+
 ## The `file_get_html` function
 
 This function is a wrapper around the [simple_html_dom](http://simplehtmldom.sourceforge.net/) [file_get_html](http://simplehtmldom.sourceforge.net/manual_api.htm#api) function in order to provide context by design. It is considered good practice to use this function.