added error and their labels page

docs/source/user_guide/error_handling.rst

@@ -0,0 +1,33 @@
+Error handling
+==============
+
+All Access Layer functions use the status codes below. The status code 
+indicates where the error occurred, while the accompanying message describes 
+the specific problem. Inspect the full status message for details.
+
+.. list-table:: Access Layer status codes
+   :header-rows: 1
+   :widths: 10 25 65
+
+   * - Code
+     - Constant
+     - Meaning
+   * - ``0``
+     - 
+     - Success
+   * - ``-1``
+     - ``UNKNOWN_ERR``
+     - Unexpected or unclassified error
+   * - ``-2``
+     - ``CONTEXT_ERR``
+     - Invalid Access Layer context or operation
+   * - ``-3``
+     - ``BACKEND_ERR``
+     - Error reported by the selected storage backend
+   * - ``-4``
+     - ``LOWLEVEL_ERR``
+     - Error in IMAS-Core processing
+
+The C API provides the numeric status code and a detailed error message. 
+Other language interfaces may return this status or report it as an
+exception.

docs/source/user_guide/index.rst

@@ -12,3 +12,4 @@ backend options, and configuration settings to help you get started with IMAS-Co
     uris_guide
     backends_guide
     configuration
+    error_handling