fix: templates source error

[cossssmin]

This makes the build error message less confusing when you didn't have a build.templates.source defined in the config.

So if you ran maizzle build imaginary but had no config.imaginary.js, you would have seen this cryptic message:

$ maizzle build imaginary
✖ Cannot read properties of undefined (reading 'source')

Now you'll get a better error message:

$ maizzle build imaginary
✖ No template sources defined in `build.templates`, check your config.imaginary.js file