Error reporting reworked

Closing resource is now parametrized by the computation status. It
enables to close resource differently for successful and failed
computations.

Result have been reworked to report all encountered errors and
exceptions.

Author: chrilves

demos/dependency_injection.py

@@ -6,7 +6,7 @@
 from typing import List
 
 
-@dataclass
+@dataclass(frozen=True)
 class NotFound(Exception):
     url: str