compileSource() fails if node modules haven't been installed, and we should fail in a graceful way

[jeffb-sfdc]

Summary

While working on #505, @klewis-sfdc discovered that if a LWC project hasn't had its dependencies installed (npm install) that when compileSource() calls transform(source, fileName, options) (in compiler.ts), that it silently fails.

It seems like this should fail, but this is a known issue and difficult to tell, so we probably should look for this condition, and then warn the user.