Add error checking to b1 and b2 files

[AMChalkie]

rMATS will run to completion with a ,, in b1/b2 files, a file not found will be reported. I would suggest that this be a fatal error that stops rMATS execution.

Final results have P.Value and FDR = 1 for all events, and the Inclusion.1 will be NA for the non-existent sample.