miniOS: Minor cleanup

Before making an actual logic change, here is some minor cleanup:

* Typo in MiniOSPatritionMismatchException: Patrition -> Partition
* PayloadGenerationSkippedException now inherits from Exception, not
  BaseException, since the Python docs say user classes should not
  inherit from BaseException. Also added to the docstring to explain the
  way it's used.
* Removed unused return value from
  paygen_payload_lib._MaybeSkipPayloadGeneration().
* In partition_lib.py, added type hints and fixed descriptive-mood
  docstring one-liners.
* Fixed a `from collections import deque`, since go/pystyle mandates we
  should use `import collections` ... `collections.deque()`.
* Pulled some logic out of a `try` block, per
  go/pystyle#exceptions-decision

BUG=b:295391636
TEST=./run_tests

Change-Id: I0f45143221c6523bc8f18e176bb32b4d3bff6a56
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4778694
Tested-by: Greg Edelston <gredelston@google.com>
Reviewed-by: Anuj Jamwal <anujjamwal@google.com>
Commit-Queue: Pin-yen Lin <treapking@chromium.org>
Auto-Submit: Greg Edelston <gredelston@google.com>
diff --git a/api/controller/payload_unittest.py b/api/controller/payload_unittest.py
index 15f32a1..ae50852 100644
--- a/api/controller/payload_unittest.py
+++ b/api/controller/payload_unittest.py
@@ -144,7 +144,7 @@
     def testNoMiniOSPartitionMismatch(self):
         """Test a miniOS paygen request with a partition count mismatch."""
         patch = self.PatchObject(paygen_payload_lib, "PaygenPayload")
-        patch.side_effect = paygen_payload_lib.MiniOSPatritionMismatchException
+        patch.side_effect = paygen_payload_lib.MiniOSPartitionMismatchException
         response_code = payload.GeneratePayload(
             self.minios_req, self.result, self.api_config
         )