huge rework; two different task pool classes now

src/asyncio_taskpool/exceptions.py

@@ -0,0 +1,26 @@
+class PoolException(Exception):
+    pass
+
+
+class PoolIsClosed(PoolException):
+    pass
+
+
+class TaskEnded(PoolException):
+    pass
+
+
+class AlreadyCancelled(TaskEnded):
+    pass
+
+
+class AlreadyFinished(TaskEnded):
+    pass
+
+
+class InvalidTaskID(PoolException):
+    pass
+
+
+class PoolStillOpen(PoolException):
+    pass