Add new exception types, and a new class for Cursor exceptions.
Fix jsdoc comments.
Add a force discard method, Cursor.discard().

Make Cursor.maybeDiscard() return false if discard was blocked and
throwing an exception was blocked too; otherwise return true.
Make NoSuchField and NoCurrentRecord extend com.qwirx.util.Exception.

To add stack trace support in V8.
getCurrentValues and getLoadedValues should not take any arguments.
Use goog.base() where possible.
Make setPosition() and moveRelative() check maybeDiscard() value.

If maybeDiscard() would have thrown an exception, but it was blocked
by event.preventDefault(), then callers need to check for this and do
nothing (return without changing anything) to allow Grid to asynchronously
make a decision on what to do with a BEFORE_DISCARD event.
Add a new class for Cursor events that affect one specific row.