Improve build validation

[carolynvs]

There are things that we could check for at build time to avoid finding out about a problem when they run the bundle. We should add a step that looks at a bundle during build and reports errors or warnings.

• Invalid templating, e.g. missing quotes, unclosed curly quotes, bundle.foo which isn't supported
• Using parameters that aren't defined or were mispelled
• Using dependencies that are not there
• Using dependency outputs from the uninstall step
• Not declaring a mixin
• etc