Informative error message when directory is not writeable

[art-dsit]

This PR contains:

• New features
• Changes to dev-tools e.g. CI config / github tooling
• Docs
• Bug fixes
• Code refactor

What is the current behavior? (You can also link to an open issue here)

You get an obscure error message if you try to run inspect in a devcontainer or other CI context without a login session.

What is the new behavior?

You get a better error message.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Other information:

[art-dsit]

(I need to test this before merging - please hold off)

[craigwalton-dsit]

LGTM, hopefully this will save some time for people!

[craigwalton-dsit]

Pedantic: consider using dedent() (used elsewhere in inspect_ai) so that the error message isn't indented

x directory x is not writeable.
        This may be because you are running Inspect without a normal login session.
        On Linux, try setting XDG_RUNTIME_DIR to somewhere writeable.
        See also https://github.com/UKGovernmentBEIS/inspect_ai/issues/51.

I.e. the following

def _xdg_error(dir_type: str, dir_name: str) -> str:
    return dedent(f"""\
        {dir_type} directory {dir_name} is not writeable.
        This may be because you are running Inspect without a normal login session.
        On Linux, try setting XDG_RUNTIME_DIR to somewhere writeable.
        See also https://github.com/UKGovernmentBEIS/inspect_ai/issues/51.""")

gives

test directory test is not writeable.
This may be because you are running Inspect without a normal login session.
On Linux, try setting XDG_RUNTIME_DIR to somewhere writeable.
See also https://github.com/UKGovernmentBEIS/inspect_ai/issues/51.

[jjallaire-aisi]

Closing as we are handling by having a more ironclad fallback in ~/.local: #761