Providing deletedAt in addition to oid in OID_NOT_FOUND error is NEO/go extension.
NEO/py sends only OidNotFound(oid):

https://lab.nexedi.com/nexedi/neoppod/blob/v1.12-0-g6332112c/neo/storage/handlers/client.py#L70-72

-> decode deletedAt only if it is present.

Since inspecting NEO/py codebase turned out that message always comes
with oid, remove the "abusing" words about this approach to convey oid
via error message for OID_NOT_FOUND and OID_DOES_NOT_EXIST.

Without this patch TestLoad was failing as

    === RUN   TestLoad/py
    I: runneo.py: /tmp/neo124851059/1: started master(s): 127.0.0.1:19681
        xtesting.go:306: load 0285cbac258bf265:0000000000000000: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbac258bf265:0000000000000000: OID_NOT_FOUND: 0000000000000000
            want: neo://1@127.0.0.1:19681: load 0285cbac258bf265:0000000000000000: 0000000000000000: object was not yet created
        xtesting.go:306: load 0285cbac3d0369e5:0000000000000001: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbac3d0369e5:0000000000000001: OID_NOT_FOUND: 0000000000000001
            want: neo://1@127.0.0.1:19681: load 0285cbac3d0369e5:0000000000000001: 0000000000000001: object was not yet created
        xtesting.go:306: load 0285cbac41b4e832:0000000000000002: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbac41b4e832:0000000000000002: OID_NOT_FOUND: 0000000000000002
            want: neo://1@127.0.0.1:19681: load 0285cbac41b4e832:0000000000000002: 0000000000000002: object was not yet created
        xtesting.go:306: load 0285cbac4666667f:0000000000000003: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbac4666667f:0000000000000003: OID_NOT_FOUND: 0000000000000003
            want: neo://1@127.0.0.1:19681: load 0285cbac4666667f:0000000000000003: 0000000000000003: object was not yet created
        xtesting.go:306: load 0285cbac4fc96318:0000000000000004: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbac4fc96318:0000000000000004: OID_NOT_FOUND: 0000000000000004
            want: neo://1@127.0.0.1:19681: load 0285cbac4fc96318:0000000000000004: 0000000000000004: object was not yet created
        xtesting.go:306: load 0285cbac547ae165:0000000000000005: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbac547ae165:0000000000000005: OID_NOT_FOUND: 0000000000000005
            want: neo://1@127.0.0.1:19681: load 0285cbac547ae165:0000000000000005: 0000000000000005: object was not yet created
        xtesting.go:306: load 0285cbac628f5c4b:0000000000000006: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbac628f5c4b:0000000000000006: OID_NOT_FOUND: 0000000000000006
            want: neo://1@127.0.0.1:19681: load 0285cbac628f5c4b:0000000000000006: 0000000000000006: object was not yet created
        xtesting.go:306: load 0285cbaca444447f:0000000000000007: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbaca444447f:0000000000000007: OID_NOT_FOUND: 0000000000000007
            want: neo://1@127.0.0.1:19681: load 0285cbaca444447f:0000000000000007: 0000000000000007: object was not yet created
        xtesting.go:306: load 0285cbacbbbbbbff:0000000000000008: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbacbbbbbbff:0000000000000008: OID_NOT_FOUND: 0000000000000008
            want: neo://1@127.0.0.1:19681: load 0285cbacbbbbbbff:0000000000000008: 0000000000000008: object was not yet created
        xtesting.go:306: load 0285cbad80da7498:0000000000000009: returned err unexpected:
            have: neo://1@127.0.0.1:19681: load 0285cbad80da7498:0000000000000009: OID_NOT_FOUND: 0000000000000009
            want: neo://1@127.0.0.1:19681: load 0285cbad80da7498:0000000000000009: 0000000000000009: object was not yet created