Better error message for missing url

[shayneo]

Is your feature request related to a problem? Please describe.

The error that occurs when making an axios request with a url that is undefined is unclear. This is problematic because I often write code that programmatically generates the url for the request, or uses the response from a previous request to compose the next request.

Describe the solution you'd like

If a url is falsey in the request config, throw a very clear error that indicates that no url was provided

Describe alternatives you've considered

Making sure I write better code that doesn't generate undefined urls

Additional context

This is the error generated currently Uncaught (in promise) TypeError: can't access property "protocol", n is undefined, which is not the most helpful in terms of pinpointing the issue.

[jasonsaayman]

Closed by #3791

[metametadata]

Reproducible in 1.4.0 (https://unpkg.com/axios@1.4.0/dist/axios.min.js). E.g. on calling axios({}):

Uncaught (in promise) TypeError: n is undefined
    we isURLSameOrigin.js:57
    xhr xhr.js:186
    xhr xhr.js:49
    Ae dispatchRequest.js:51
    value Axios.js:148
    a bind.js:5
    <anonymous> debugger eval code:1