friendly error message when negcon params are all FALSE

[allonw]

When negcon_ruv or negcon_eval in SconeExperiment() are all FALSE due to a bug, then the scone() function with norm_adjust_batch throws the error

Error in svd(Ycenter[, cIdx]) : a dimension is zero

Instead, it could give a helpful error msg. Other users had similar issues in RUVSeq see:
https://support.bioconductor.org/p/70281/