Use better storage error in tests

[sisuresh]

@marta-lokhova ran into this error in a test case when trying to retrieve data that didn't exist -

Value: Status(HostStorageError(UnknownError))

Debug events (newest first):
   0: "escalating error 'Status(HostStorageError(UnknownError))' to panic"
   1: "contract call invocation resulted in error Status(HostStorageError(UnknownError))"
   2: "caught panic from contract function 'Symbol(attend)', propagating escalated error 'Status(HostStorageError(UnknownError))'"
   3: "escalating error 'Status(HostStorageError(UnknownError))' to panic"
   10: ... elided ...

This error doesn't mention the issue, which is that the key doesn't exist. With the change in this PR, the error will look like -

Value: Status(HostStorageError(MissingKeyInGet))

Debug events (newest first):
   0: "escalating error 'Status(HostStorageError(MissingKeyInGet))' to panic"
   1: "contract call invocation resulted in error Status(HostStorageError(MissingKeyInGet))"
   2: "caught panic from contract function 'Symbol(get)', propagating escalated error 'Status(HostStorageError(MissingKeyInGet))'"
   3: "escalating error 'Status(HostStorageError(MissingKeyInGet))' to panic"