Error when using unsupported NDK directory

[keith]

Currently if you attempt to use these rules with NDK 21, you get cryptic failures because of broken assumptions. This is expected to fail in general but it would be nicer if the rules reported a clear error for this case instead

[ahumesky]

the native version of android_ndk_repository reads source.properties:
https://github.com/bazelbuild/bazel/blob/818c5c8693c43fe490c9f6b2c05149eb8f45cf52/src/main/java/com/google/devtools/build/lib/bazel/rules/android/AndroidNdkRepositoryFunction.java#L535

it would probably be pretty straightforward to do something similar in the Starlark version