Add more comprehensive error message to dna_network when all documents are excluded

[JBGruber]

Code for reproducing the error:

library("rDNA")
dna_init("dna-2.0-beta20.jar")
conn <- dna_connection("sample.dna")

congruence <- dna_network(conn, 
                          networkType = "onemode", 
                          statementType = "DNA Statement", 
                          timewindow = "days", 
                          windowsize = 11,
                          duplicates = "document",
                          excludeSources = "109")

Error message reads:
0 out of 41 statements retained. Error in .jcall(connection$dna_connection, "[J", "getTimeWindowTimes", : java.lang.IndexOutOfBoundsException: Index: 0, Size: 0

Should be something more comprehensible like:
"Your exclude options removed all documents while retrieving a network. Nothing left to compute.".