🚀 Feature: Show a meaningful error when the HTML reporter is used via the CLI

[thisismydesign]

Description

$ npx mocha --opts ./mocha.opts --reporter=html
document is not defined

The real issue here is that html reporter is not even an option from the CLI.

$ npx mocha --reporters

    doc         - HTML documentation
    dot         - dot matrix representation
    json        - single JSON object
    json-stream - newline delimited JSON events
    landing     - Unicode landing strip
    list        - like "spec" reporter but flat
    markdown    - GitHub Flavored Markdown
    min         - essentially just a summary
    nyan        - "nyan cat"
    progress    - a progress bar
    spec        - hierarchical & verbose [default]
    tap         - TAP-compatible output
    xunit       - XUnit-compatible XML output

Using the HTML reporter is recommended in some places without context (e.g. https://stackoverflow.com/a/26635182/2771889).

There seem to be quite a few people confused by this:

• https://stackoverflow.com/questions/32794700/running-mocha-test-but-getting-error-referenceerror-document-is-not-defined
• google mocha html report ReferenceError: document is not defined

Versions

6.2.0

[thisismydesign]

On second look, this seems to only be an issue with the html reporter. Otherwise, I get Unknown "reporter": htmllll.

[juergba]

HTML is a valid reporter, but not intended for CL usage, see docs

related: #1906

[thisismydesign]

HTML is a valid reporter, but not intended for CL usage, see docs

So doesn't that make it invalid in that context? We can argue about the wording but I trust that you get my point..