[SPARK-17748][ML] Minor cleanups to one-pass linear regression with elastic net

## What changes were proposed in this pull request?

* Made SingularMatrixException private ml
* WeightedLeastSquares: Changed to allow tol >= 0 instead of only tol > 0

## How was this patch tested?

existing tests

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #15779 from jkbradley/wls-cleanups.

(cherry picked from commit 26e1c53a)
Signed-off-by: Joseph K. Bradley <joseph@databricks.com>