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.