Graceful exception for ExpandAsColumns

[sylvainhalle]

ExpandAsColumn throws an ugly exception when the user refers to a column that does not exist:

java.lang.ArrayIndexOutOfBoundsException: 4
	at ca.uqac.lif.mtnp.table.ExpandAsColumns.transform(ExpandAsColumns.java:113)
	at ca.uqac.lif.mtnp.table.TransformedTable.getDataTable(TransformedTable.java:78)
	at ca.uqac.lif.mtnp.table.Table.getDataTable(Table.java:229)
	at ca.uqac.lif.labpal.server.TablePageCallback.fill(TablePageCallback.java:85)
	at ca.uqac.lif.labpal.server.TemplatePageCallback.render(TemplatePageCallback.java:153)
	at ca.uqac.lif.labpal.server.TemplatePageCallback.render(TemplatePageCallback.java:147)
	at ca.uqac.lif.labpal.server.TemplatePageCallback.process(TemplatePageCallback.java:104)
	at ca.uqac.lif.jerrydog.Server.handle(Server.java:252)
	at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:79)
	at sun.net.httpserver.AuthFilter.doFilter(AuthFilter.java:83)
	at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:82)
	at sun.net.httpserver.ServerImpl$Exchange$LinkHandler.handle(ServerImpl.java:675)
	at com.sun.net.httpserver.Filter$Chain.doFilter(Filter.java:79)
	at sun.net.httpserver.ServerImpl$Exchange.run(ServerImpl.java:647)
	at sun.net.httpserver.ServerImpl$DefaultExecutor.execute(ServerImpl.java:158)
	at sun.net.httpserver.ServerImpl$Dispatcher.handle(ServerImpl.java:431)
	at sun.net.httpserver.ServerImpl$Dispatcher.run(ServerImpl.java:396)
	at java.lang.Thread.run(Thread.java:748)

The code should trap this and re-throw a better kind of Exception.