Fix: control stringification of error message

[syeutyu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions.

Description of the Change

Before, The order of the previous processing is as follows

1. error message is function
2. error include inspect parameter
3. empty

After

1. check include inspect custom function
2. error message is function
3. empty

So developers can custom error message if they want

I have been found util.inspect.custom API, but if set up util.inspect.custom that changed error format

So this pr only change ordering with err.inspect function and err.message 😭

Benefits

Developer can custom error message handling in mocha

Applicable issues

issues:3985

[jsf-clabot]

All committers have signed the CLA.

[coveralls]

Coverage remained the same at 94.273% when pulling 3b7edd9 on syeutyu:issue-3985 into 1b5cbf1 on mochajs:master.

[juergba]

@syeutyu could you rebase to master, please?
I would like to review and then merge for the next release, thank you.

[syeutyu]

I have rebased it @juergba

[juergba]

@syeutyu thank you for this PR.
I just have two small changes, then it's ready to get merged.

[syeutyu]

@juergba I've updated it.
Thank you for your comment 😄

[juergba]

@syeutyu thank you