Remote Execution Status messages embedded in ExecuteResponses are
extremely capable vehicles for conveying the nature of an error, and
informing a user of further steps to take to remediate it. This change
expands the presentation of these response Statuses, and brings all of
the error details to light, by default instead of requiring
--verbose_failures to investigate any details of a remote execution
problem.

The interpretation of precondition failures to highlight retriable
responses has been expanded to ignore benign details that might be
included in a response.

SpawnResult error message composition has been simplified substantially,
without any special behavior for 'Remote' errors, and a removal of a
duplicate message printout incurred in the wake of succcessive @janakr
and @olaola changes. Failure messages are now implied to be present in
all spawn result failure reporting exactly once, and the failureMessage
of a SpawnResult is implied to be the parameter to getDetailMessage.

An example error presentation is as follows (including the modifications
to SpawnResult's output formatting):

```
ERROR: /home/werkt/dev/test/BUILD:22:10: Linking test failed: (Exit 34): Remote Execution Failure:
Failed Precondition: Action 4223ab2cc114385110714243a0b4a88cc743f2169b5be7d4d438a6bbba4f529f/142 is invalid
  Resource Info: type.googleapis.com/google.longrunning.Operation: name='shard/operations/9335fef2-184b-4d26-9a6f-2f27cebe7527', owner='tool_invocation_id:4b4bf7b1-fadd-44fd-99be-a234e7c26fc4,correlated_invocation_id:dc88325a-9317-48c0-9013-b3bb8b7a458f'
  Precondition Failure:
    (MISSING) bazel-out/k8-fastbuild/bin/test: 7872: An output could not be uploaded because it exceeded the maximum size of an entry
Target //:test failed to build
```

When a retryInfo is supplied, it should circumvent any other conditions
which would prevent retriability. Its delay will inform the subsequent
backoff delay supplied, assuming it is not beyond the retry count.