Fix err checks from volume manager

What does this MR do?

Fix err checks from volume manager

Why was this MR needed?

In !1989 (merged) we started wrapping the errors with the new Go 1.13 syntax. What we failed to update is the assertions, we still used the old assertions such as == and using type assertion which don't work when the errors are wrapped. Using errors.Is and errors.As.

The tests didn't catch this because we mock the errors and we just return the error and don't wrap it with %w like it is done in the actual implement so the old assertions worked fine.

Testing

config.toml

[[runners]]
  name = "docker"
  url = "http://192.168.144.160:3000"
  token = "xxxx"
  executor = "docker"
  [runners.docker]
    tls_verify = false
    image = "alpine:3.11"
    privileged = true
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = true
    volumes = ["/cache", "/tmp/builds:/builds" ]
    shm_size = 0
    helper_image = "gitlab/gitlab-runner-helper:x86_64-4c96e5ad"

master branch

now

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #25428 (closed)