uerrno: Document alongside OSError.

These are meant to be used together, so document them in the same place along with a practical example.

doc/main/micropython/exceptions.rst

@@ -33,8 +33,6 @@ Exceptions and error codes
 
 .. autoclass:: ubuiltins.NotImplementedError
 
-.. autoclass:: ubuiltins.OSError
-
 .. autoclass:: ubuiltins.OverflowError
 
 .. autoclass:: ubuiltins.RuntimeError
@@ -51,8 +49,19 @@ Exceptions and error codes
 
 .. autoclass:: ubuiltins.ZeroDivisionError
 
-.. automodule:: uerrno
-   :no-members:
+OSError
+--------
+
+.. autoclass:: ubuiltins.OSError
+
+
+.. rubric:: uerrno module
+
+.. module:: uerrno
+
+The ``errno`` attribute of an ``OSError`` indicates why the exception was
+raised. It has one of the following values, which can be imported from the
+``uerrno`` module:
 
 .. autodata:: uerrno.EAGAIN
 
@@ -73,3 +82,12 @@ Exceptions and error codes
 .. autodata:: uerrno.ETIMEDOUT
 
 .. autodata:: uerrno.errorcode
+
+Examples
+---------------------
+
+Detect devices using ``OSError``
+*****************************************
+
+.. literalinclude::
+    ../../../examples/micropython/oserror.py

examples/micropython/oserror.py

@@ -0,0 +1,11 @@
+from pybricks.pupdevices import Motor
+from pybricks.parameters import Port
+
+from uerrno import ENODEV
+
+try:
+    my_motor = Motor(Port.A)
+    print("Detected a motor.")
+except OSError as ex:
+    if ex.errno == ENODEV:
+        print("There is no motor this port.")

src/ubuiltins/__init__.py

@@ -967,15 +967,15 @@ class NotImplementedError(RuntimeError):
 
 class OSError(Exception):
     """
-    This exception is raised when a system function returns a system-related
-    error, including I/O failures.
+    This exception is raised by the firmware, which is
+    the Operating System that runs on the hub. For example, it
+    raises an ``OSError`` if you call ``Motor(Port.A)`` when there is no
+    motor on port A.
     """
 
     errno: _int
     """
-    A numeric error code.
-
-    Also see :mod:`uerrno`.
+    Specifies which kind of ``OSError`` occurred, as listed below.
     """
 
 
@@ -1010,8 +1010,7 @@ class SyntaxError(Exception):
 
 class SystemExit(BaseException):
     """
-    This exception is raised when the stop button is pressed (on the hub or in
-    the IDE).
+    Raised when you press the stop button on the hub or in the Pybricks Code app.
     """
 
 

src/uerrno/__init__.py

@@ -61,9 +61,5 @@ The operation timed out.
 
 errorcode: Dict[int, str]
 """
-Dictionary mapping numeric error codes to strings with symbolic error
-code (see above)::
-
-    >>> print(uerrno.errorcode[uerrno.EEXIST])
-    EEXIST
+Dictionary that maps numeric error codes to strings with symbolic error code.
 """