cli: make cli.StatusError slightly prettier

[thaJeztah]

• relates to cmd/docker: split handling exit-code to a separate utility #5229 (comment)

This error didn't do a great job at formatting. If a StatusError was produced without a Status message, it would print a very non-informative error, with information missing.

Let's update the output:

• If a status-message is provided; print just that (after all the status code is something that can be found from the shell, e.g. through echo $? in Bash).
• If no status-message is provided: print a message more similar to Go's exec.ExecError, which uses os.rocessState.String() (see 1).

Before this patch, an error without custom status would print:

Status: , Code: 2

After this patch:

exit status 2

In situations where a custom error-message is provided, the error-message
is print as-is, whereas before this patch, the message got combined with
the Status: and Code:, which resulted in some odd output.

Before this patch:

docker volume --no-such-flag
Status: unknown flag: --no-such-flag
See 'docker volume --help'.

Usage:  docker volume COMMAND

Manage volumes

Commands:
  create      Create a volume
  inspect     Display detailed information on one or more volumes
  ls          List volumes
  prune       Remove unused local volumes
  rm          Remove one or more volumes
  update      Update a volume (cluster volumes only)

Run 'docker volume COMMAND --help' for more information on a command.
, Code: 125

With this patch, the error is shown as-is;

docker volume --no-such-flag
unknown flag: --no-such-flag
See 'docker volume --help'.

Usage:  docker volume COMMAND

Manage volumes

Commands:
  create      Create a volume
  inspect     Display detailed information on one or more volumes
  ls          List volumes
  prune       Remove unused local volumes
  rm          Remove one or more volumes
  update      Update a volume (cluster volumes only)

Run 'docker volume COMMAND --help' for more information on a command.

While the exit-code is no longer printed, it's still properly handled;

echo $?
125

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 61.47%. Comparing base (5aae44b) to head (bca2090).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5231      +/-   ##
==========================================
- Coverage   61.48%   61.47%   -0.02%     
==========================================
  Files         298      298              
  Lines       20813    20815       +2     
==========================================
- Hits        12797    12795       -2     
- Misses       7104     7107       +3     
- Partials      912      913       +1     

[thaJeztah]

I have some changes staged to look at actually providing an error-message for some of these, but I need to clean it up a bit 😅 - also have #5229 (comment) as a follow-up.