#### Describe the bug
The irrelevant error returned when requesting illegal URL, like: "tel:484-695-3408"
```
TypeError: Cannot read property 'replace' of null
    at dispatchHttpRequest (node_modules/axios/lib/adapters/http.js:109:74)
    at new Promise (<anonymous>)
    at httpAdapter (node_modules/axios/lib/adapters/http.js:46:10)
    at dispatchRequest (node_modules/axios/lib/core/dispatchRequest.js:52:10)
```

#### To Reproduce
```js
axios.get('tel:484-695-3408').catch(err => console.error(err));
```

#### Expected behavior
Return descriptive error, like "Unsupported protocol tel:"

#### Environment
 - Axios Version: 0.21.1
 - Adapter: HTTP
 - Browser: none
 - Browser Version: none
 - Node.js Version: 12.19.0
 - OS: MacOS 10.14
 - Additional Library Versions: none

#### Additional context/Screenshots
none
lib/adapters/xhr.js
request = null;
    };
    
    request.onerror = function handleError() {
      
      
      reject(createError('Network Error', config, null, request));
      
      request = null;
    };
    
    request.ontimeout = function handleTimeout() {
      var timeoutErrorMessage = config.timeout ? 'timeout of ' + config.timeout + 'ms exceeded' : 'timeout exceeded';
      var transitional = config.transitional || transitionalDefaults;
      if (config.timeoutErrorMessage) {
        timeoutErrorMessage = config.timeoutErrorMessage;
      }
      reject(createError(
        timeoutErrorMessage,
        config,
        transitional.clarifyTimeoutError ? 'ETIMEDOUT' : 'ECONNABORTED',
        request));
      
      request = null;
    };
    
    
    
    if (utils.isStandardBrowserEnv()) {