Improve error messages when an operation results in 0 samples or taxa

[mikemc]

Currently if one performs a subset_samples operation that removes all samples, they get an error message

Error in validObject(.Object) :
  invalid class “sample_data” object: Sample Data must have non-zero dimensions. 

Instead, we should detect that 0 samples pass the subsetting operation and inform the user in an error message, to help spot the problem. Similarly for subsetting taxa with subset_taxa.