Improve error messages when dependency variants are missing

Instead of just saying:

error: .../Android.bp:48:1: dependency "libc++" of "libtest" missing variant "arch:android_arm_armv7-a, link:shared, vndk:"

Include a list of currently existing variants:

error: .../Android.bp:48:1: dependency "libc++" of "libtest" missing variant:
  arch:android_arm_armv7-a, link:shared, vndk:
available variants:
  arch:android_arm_armv7-a, link:shared
  arch:linux_x86, link:shared
  arch:linux_x86_64, link:shared
  arch:windows_x86, link:shared
  arch:windows_x86_64, link:shared

This still isn't the best experience for users, but it at least provides
enough information for someone more familiar with the build to
understand the problem.