I'm overriding SetValue in my custom cell, to parse the new value and decide
what value type it is, e.g. if it starts with #, then it is a string
literal, otherwise it is a field name.  Now I want to handle aggregate
functions as well, so I need to parse for, e.g.  Sum('xxx'), where 'xxx' is
a field name.  How do I cancel the edit if the input is of bad aggregate
syntax, or a non-defined function, such as MyFunc('xxx')?  Do I throw an
exception in SetValue, catch it, and cancel the edit event?  How is this
normally handled in bound grids?