Adding exist checks for input file arguments, fixes #61

Added exist checks for both bam file and genomic region file arguments
```bash
$ ngs-filter-consensus -x foo.txt -i not-a-file -g HLA-A
usage:
ngs-filter-consensus -i input.bam -x genome.txt -g HLA-A [args]

java.lang.IllegalArgumentException: -i, --input-bam-file must be a file that exists
	at org.nmdp.ngs.tools.FilterConsensus.main(FilterConsensus.java:353)
...

$ ngs-filter-consensus -x not-a-file -i foo.bam -g HLA-A
usage:
ngs-filter-consensus -i input.bam -x genome.txt -g HLA-A [args]

java.lang.IllegalArgumentException: -x, --input-genomic-range-file must be a file that exists
	at org.nmdp.ngs.tools.FilterConsensus.main(FilterConsensus.java:356)
...
```