DimensionApiRequestMapper should throw contracted exception

[QubitPi]

While running a debugging session, I accidentally noticed this line of code:

https://github.com/yahoo/fili/blob/master/fili-core/src/main/java/com/yahoo/bard/webservice/web/DimensionApiRequestMapper.java#L62

By interface contract, apply should throw RequestValidationException instead of BadApiRequestException

Another consequence is a bug where dimension servlet won't be able to catch error from requestMapper.apply(apiRequest, containerRequestContext):

https://github.com/yahoo/fili/blob/master/fili-core/src/main/java/com/yahoo/bard/webservice/web/endpoints/DimensionsServlet.java#L156