Rewrite the exc module docs

Revise the module docs and rewrite the docs for the `ConflictError` and `NotFoundError`
classes.

Refs #10

alchemical_storage/storage/exc.py

@@ -1,9 +1,9 @@
-"""Exceptions for alchemical_storage."""
+"""Exceptions for the ``alchemical_storage.storage`` package."""
 
 
 class ConflictError(Exception):
-    """Conflict error."""
+    """Raised when putting a value that already exists in the storage."""
 
 
 class NotFoundError(Exception):
-    """Not found error."""
+    """Raised when getting a value that does not exist in the storage."""