warning msg and ignore-response for attempted mapping of discrete var w/ too many classes

[joey711]

Need a warning message and an ignore-response for this problem of attempting to map a discrete variable with too many categories to an aesthetic that cannot support it. The message let's the user know that they have selected a variable that will not map well to color or shape, while the ignore-response avoids the computationally expensive and confusing task of attempting to perform the aesthetic mapping in the first place.