docs,tests: Add docs and test for uasyncio custom exc handler methods.

docs/library/uasyncio.rst

@@ -266,3 +266,22 @@ Event Loop
 .. method:: Loop.close()
 
     Close the event loop.
+
+.. method:: Loop.set_exception_handler(handler)
+
+    Set the exception handler to call when a Task raises an exception that is not
+    caught.  The *handler* should accept two arguments: ``(loop, context)``.
+
+.. method:: Loop.get_exception_handler()
+
+    Get the current exception handler.  Returns the handler, or ``None`` if no
+    custom handler is set.
+
+.. method:: Loop.default_exception_handler(context)
+
+    The default exception handler that is called.
+
+.. method:: Loop.call_exception_handler(context)
+
+    Call the current exception handler.  The argument *context* is passed through and
+    is a dictionary containing keys: ``'message'``, ``'exception'``, ``'future'``.

tests/extmod/uasyncio_set_exception_handler.py

@@ -0,0 +1,46 @@
+# Test that tasks return their value correctly to the caller
+
+try:
+    import uasyncio as asyncio
+except ImportError:
+    try:
+        import asyncio
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+
+
+def custom_handler(loop, context):
+    print("custom_handler", repr(context["exception"]))
+
+
+async def task(i):
+    # Raise with 2 args so exception prints the same in uPy and CPython
+    raise ValueError(i, i + 1)
+
+
+async def main():
+    loop = asyncio.get_event_loop()
+
+    # Check default exception handler, should be None
+    print(loop.get_exception_handler())
+
+    # Set exception handler and test it was set
+    loop.set_exception_handler(custom_handler)
+    print(loop.get_exception_handler() == custom_handler)
+
+    # Create a task that raises and uses the custom exception handler
+    asyncio.create_task(task(0))
+    print("sleep")
+    await asyncio.sleep(0)
+
+    # Create 2 tasks to test order of printing exception
+    asyncio.create_task(task(1))
+    asyncio.create_task(task(2))
+    print("sleep")
+    await asyncio.sleep(0)
+
+    print("done")
+
+
+asyncio.run(main())

tests/extmod/uasyncio_set_exception_handler.py.exp

@@ -0,0 +1,8 @@
+None
+True
+sleep
+custom_handler ValueError(0, 1)
+sleep
+custom_handler ValueError(1, 2)
+custom_handler ValueError(2, 3)
+done