Clarify what a client should do on FAILED_PRECONDITION

As discussed with @ola-rozenfeld, we need to clarify what the client should do in case of FAILED_PRECONDITION

https://github.com/bazelbuild/remote-apis/blob/master/build/bazel/remote/execution/v2/remote_execution.proto#L101

In particular for any blobs that are reported as MISSING, the client should, if a blob is a Directory, ensure that the entire tree rooted at the directory is going to be present in CAS prior to re-submission. This practically involves a call to FindMissingBlobs for anything transiently referred to by the directory.

Start at build/bazel/remote/execution/v2/remote_execution.proto near line 101 and review the FAILED_PRECONDITION guidance. Check how MISSING blobs and FindMissingBlobs are described, then clarify that a client must ensure a missing Directory's entire tree is in CAS before resubmitting. Done means the protocol documentation unambiguously explains this behavior.

Written by the indexing model from the issue text.