add warning around opening suspicious ADAM files

I just got bit by trying to open a path that was a directory (I meant to pass a BAM); failure mode was not the most clear:

```
Exception in thread "Driver" java.io.IOException: Could not read footer: java.lang.RuntimeException: hdfs://demeter-nn1.demeter.hpc.mssm.edu:8020/user/willir31/data/set3/normal/set3.normal.fq/part-01487 is not a Parquet file. expected magic number at tail [80, 65, 82, 49] but found [67, 68, 67, 10]
	at parquet.hadoop.ParquetFileReader.readAllFootersInParallel(ParquetFileReader.java:238)
	at parquet.hadoop.ParquetFileReader.readAllFootersInParallelUsingSummaryFiles(ParquetFileReader.java:179)
	at parquet.hadoop.ParquetInputFormat.getFooters(ParquetInputFormat.java:389)
	at parquet.hadoop.ParquetInputFormat.getFooters(ParquetInputFormat.java:361)
	at parquet.hadoop.ParquetInputFormat.getSplits(ParquetInputFormat.java:245)
```

Something like this maybe makes sense; what file extensions are common for ADAM files?