Adding git based platform does give unhelpful error on system with no git installed

# Bug Report

## Problem

### What is expected to happen?

User should be told that they are missing git and should install it.

### What does actually happen?

```
C:\Users\User\Documents\cordovaWindows>cordova platform add https://github.com/apache/cordova-windows
Using cordova-fetch for https://github.com/apache/cordova-windows
Failed to fetch platform https://github.com/apache/cordova-windows
Probably this is either a connection problem, or platform spec is incorrect.
Check your connection and platform name/version/URL.
Error: npm: Command failed with exit code 1 Error output:
npm ERR! path git
npm ERR! code ENOENT
npm ERR! errno ENOENT
npm ERR! syscall spawn git
npm ERR! enoent Error while executing:
npm ERR! enoent undefined ls-remote -h -t https://github.com/apache/cordova-windows.git
npm ERR! enoent
npm ERR! enoent
npm ERR! enoent spawn git ENOENT
npm ERR! enoent This is related to npm not being able to find a file.
npm ERR! enoent

npm ERR! A complete log of this run can be found in:
npm ERR! C:\Users\User\AppData\Roaming\npm-cache\_logs\2019-05-04T19_00_14_466Z-debug.log
```

## Information

Add a platform via its git URL (e.g. GitHub) on a system that has no git installed.

## Checklist

- [x] I searched for existing GitHub issues
- [x] I updated all Cordova tooling to most recent version
- [x] I included all the necessary information above

Start with the `cordova platform add ` entry point and reproduce the failure on a system without git installed. Trace the npm git-fetch error and make the user-facing result identify the missing git dependency and installation need; done means this case no longer reports only a generic connection or platform-specification error.

Written by the indexing model from the issue text.