Provide helpful error message when requested dependency is not found

[rpkyle]

As described by @KPhans, get_package_mapping() throws a bit of an obscure error when dependency resolution fails. This probably occurs when the dependency name is not matched in the package_map object:

Error in get_package_mapping: 'package' must be of length 1'.

We should return a user-friendly error message here, so that app users understand that the problem is dependency-related, and so that app and component library developers can troubleshoot more easily during their build process.