WIP: #4452 Add message specify_file_by_relative_path

[clarmso]

• Closes On cy.visit('file://...) throw err to explain they should visit relative html file #4452

Pre-merge Tasks

• Have tests been added/updated for the changes in this PR? (TODO)
• Has a PR to cypress-documentation been submitted to document any user-facing changes Add note on url for cy.visit cypress-documentation#1989

I've added an error message notifying the user that the "file:///" protocol isn't supported by Cypress. I'm marking this PR as a WIP because

• The error message can be improved.
• The documentation to cy.visit() does not specify that the "file:///" protocol isn't supported. I need to add a note to this page.
• Where should I add the test? I cannot add it to navigation_spec.coffee because of the same origin policy. In other words, I cannot just have a test with a single cy.visit('file:///....') command. The cypress.json file has the baseUrl specified.

[CLAassistant]

All committers have signed the CLA.

[jennifer-shehane]

Thanks for the contribution! Could you please sign our CLA?

[clarmso]

@jennifer-shehane Let me tried to fix the github email address. I've signed the CLA using the link provided by CLAassistant's link above. CLA signed!

[clarmso]

Bummer. I merged someone unrelated to my work by mistake. Here's the PR: #4982