The `reportError` utility was present because cli.StatusError would print
the error decorated with `Status: <error-message>, Code: <exit-code>`.
That was not desirable in many cases as it would mess-up the output. To
prevent this, the CLI had code to check for an empty `Status` (error message)
in which case the error would be "ignored" (and only used for the exit-status),
and the `reportError` utility would be used to manually print a custom error
message before returning the error.

Now that bca2090 fixed the output format
of `cli.StatusError`, and 3dd6fc3 and
350a0b6 no longer discard these error,
we can get rid of this utility, and just set the error-message for
the status-error.

This patch:

- Introduces a `withHelp` which takes care of decorating errors with
  a "Run --help" hint for the user.
- Introduces a `toStatusError` utility that detects certain errors in
  the container to assign a corresponding exit-code (these error-codes
  can be used to distinguish "client" errors from "container" errors).
- Removes the `reportError` utility, and removes code that manually
  printed errors before returning.

Behavior is mostly unmodified, with the exception of some slight reformatting
of the errors:

- `withHelp` adds a `docker:` prefix to the error, to indicate the error
  is produced by the `docker` command. This prefix was already present
  in most cases.
- The "--help" hint is slightly updated ("Run 'docker run --help' for
  more information" instead of "See 'docker run --help'"), to make it
  more clear that it's a "call to action".
- An empty is added before the "--help" hint to separate it better from
  the error-message.

Before this patch:

    $ docker run --pull=invalid-option alpine
    docker: invalid pull option: 'invalid-option': must be one of "always", "missing" or "never".
    See 'docker run --help'.
    $ echo $?
    125

    $ docker run --rm alpine nosuchcommand
    docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: exec: "nosuchcommand": executable file not found in $PATH: unknown.
    $ echo $?
    127

With this patch:

    $ docker run --pull=invalid-option alpine
    docker: invalid pull option: 'invalid-option': must be one of "always", "missing" or "never"

    Run 'docker run --help' for more information
    $ echo $?
    125

    $ docker run --rm alpine nosuchcommand
    docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: exec: "nosuchcommand": executable file not found in $PATH: unknown.

    Run 'docker run --help' for more information
    $ echo $?
    127

Signed-off-by: Sebastiaan van Stijn <[email protected]>