docs/uerrno: Document "uerrno" module.

docs/library/index.rst

@@ -77,6 +77,7 @@ it will fallback to loading the built-in ``ujson`` module.
        sys.rst
        ubinascii.rst
        ucollections.rst
+       uerrno.rst
        uhashlib.rst
        uheapq.rst
        uio.rst
@@ -102,6 +103,7 @@ it will fallback to loading the built-in ``ujson`` module.
        sys.rst
        ubinascii.rst
        ucollections.rst
+       uerrno.rst
        uhashlib.rst
        uheapq.rst
        uio.rst
@@ -144,6 +146,7 @@ it will fallback to loading the built-in ``ujson`` module.
        sys.rst
        ubinascii.rst
        ucollections.rst
+       uerrno.rst
        uhashlib.rst
        uheapq.rst
        uio.rst

docs/library/uerrno.rst

@@ -0,0 +1,34 @@
+:mod:`uerrno` -- system error codes
+===================================
+
+.. module:: uerrno
+   :synopsis: system error codes
+
+|see_cpython_module| :mod:`python:errno`.
+
+This module provides access to symbolic error codes for `OSError` exception.
+A particular inventory of codes depends on `MicroPython port`.
+
+Constants
+---------
+
+.. data:: EEXIST, EAGAIN, etc.
+
+    Error codes, based on ANSI C/POSIX standard. All error codes start with
+    "E". As mentioned above, inventory of the codes depends on
+    `MicroPython port`. Errors are usually accessible as ``exc.args[0]``
+    where `exc` is an instance of `OSError`. Usage example::
+
+        try:
+            uos.mkdir("my_dir")
+        except OSError as exc:
+            if exc.args[0] == uerrno.EEXIST:
+                print("Directory already exists")
+
+.. data:: errorcode
+
+    Dictionary mapping numeric error codes to strings with symbolic error
+    code (see above)::
+
+        >>> print(uerrno.errorcode[uerrno.EEXIST])
+        EEXIST