docs/uerrno: Document "uerrno" module.

Author: pfalcon

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