Added filenames to "Unmatched SAI magic" message

This was helpful for me debugging a problem so I thought others may find it useful. I realize that the loop over two files may seem silly to some, but it made sense from a "DRY" perspective since I wanted to output which file did not have the magic number.

Here's an example:
```
# dd if=/dev/urandom of=./chr1_2.sai count=81 bs=1K
# ~/bwa/bwa sampe ./chr1.fa chr1_1.sai chr1_2.sai small1.fq small2.fq > bad.sam
[E::bwa_sai2sam_pe_core] Unmatched SAI magic in file "chr1_2.sai". Please re-run `aln' with the same version of bwa.
```