Resolve relative Spa Source based on Content Root

[pardahlman]

This PR closes #1678 by evaluating the provided SourcePath and doing the following:

1. Detect if path is relative and if so make it relative to the content root (with fallback to current directory)
2. Evaluate if the now absolute path exists and if not, throw a helpful exception
3. Using the existing, absolute path to start the NpmScriptRunner

[dnfclas]

All CLA requirements met.

[Eilon]

Hi, this pull request is being closed because this repo is archived and all the source code and issues have been moved to the https://github.com/aspnet/AspNetCore repository. If you would like to send a new PR, you can find the new location for the source code here: https://github.com/aspnet/AspNetCore/tree/master/src/Middleware/NodeServices

Thanks,
Eilon