ENH: drop function now has errors keyword for non-existing column handling

Currently `drop` raises `ValueError` when non-existing label is passed. I think it is useful if `drop` has an option to suppress error and drop existing labels only.
 
For example, I sometimes process lots of files which has slightly different columns, and want to `drop` if data has unnecessary columns. Previously, I have to prepare different `drop` arguments checking columns existing each data.