Throw explicit exception if experimental_split_coverage_postprocessing is used without experimental_fetch_all_coverage_outputs

[blindpirate]

This closes https://github.com/bazelbuild/bazel/issues/13185 According to https://github.com/bazelbuild/bazel/commit/e411fa74176cdc54f84d62c189d042e9fb6d5c79, The flag --experimental_split_coverage_postprocessing depends on the flag --experimental_fetch_all_coverage_outputs being enabled, but if the former one is set without the latter one, Bazel throws a quite confusing NullPointerException. Now we throw an explicit exception with proper error message.

[sgowroji]

Hello @blindpirate, Can you please check build failures. Thank you!

[blindpirate]

@sgowroji The failure seems to be because CI machine is disconnected from Internet, can you give me some hints how to handle external dependencies which need to be downloaded in integration tests?

[sgowroji]

Hello @oquenchil, Could you please guide on the above buildkite check failures. Thanks!

[oquenchil]

Adding @c-mita

[blindpirate]

Hi @sgowroji @oquenchil @c-mita now it's all green, PTAL!

[sgowroji]

Thank you @blindpirate !