Provide better sandboxfs errors (e.g. when FUSE isn't available).

[PiotrSikora]

Description of the problem / feature request:

sandboxfs doesn't provide helpful errors (e.g. when FUSE isn't available) and returns generic sandboxfs failed to start error.

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

$ sudo mv /bin/fusermount /tmp/; bazel build --experimental_use_sandboxfs //...; sudo mv /tmp/fusermount /bin
INFO: Mounting sandboxfs instance on <redacted>/sandbox/sandboxfs
ERROR: Failed to initialize sandbox: sandboxfs failed to start
INFO: Elapsed time: 0.204s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded)

What operating system are you running Bazel on?

$ uname -a
Linux buildbox 4.9.0-6-amd64 #1 SMP Debian 4.9.88-1+deb9u1 (2018-05-07) x86_64 GNU/Linux
$ cat /etc/debian_version
9.4

What's the output of bazel info release?

$ bazel info release
release 0.14.1

[jmmv]

Just to pile onto the bug because this is related: depending on how sandboxfs fails, Blaze may even report that "sandboxed" is an unknown strategy. I think this happens when the sandboxfs binary cannot be found, but I forget now.

[github-actions]

Thank you for contributing to the Bazel repository! This issue has been marked as stale since it has not had any activity in the last 2.5 years. It will be closed in the next 14 days unless any other activity occurs or one of the following labels is added: "not stale", "awaiting-bazeler". Please reach out to the triage team (@bazelbuild/triage) if you think this issue is still relevant or you are interested in getting the issue resolved.

[github-actions]

This issue has been automatically closed due to inactivity. If you're still interested in pursuing this, please reach out to the triage team (@bazelbuild/triage). Thanks!