Raw Feature Filter should throw exception when removing all features

[crupley]

Problem
Currently, the Raw Feature Filter can remove all features and there is no way to communicate that signal in the pipeline. This is different for how the same situation is handled in the Sanity Checker. See:

TransmogrifAI/core/src/main/scala/com/salesforce/op/stages/impl/preparators/SanityChecker.scala

Lines 683 to 684 in 3e9a815

	require(indicesToKeep.length > 0,
	"The sanity checker has dropped all of your features, check your input data quality")

Solution
Require that the raw feature filter return at least one feature after filtering so that behavior is consistent with similar feature filtering stages.

Alternatives
An alternative would be to throw no exception in either case and require all stages to support zero-length vectors.

Additional context
Nah

[crupley]

Feature added