make error msg more descriptive

Compare:

```
$> PYTHONPATH=$PWD bin/fingerprint -d  ./tests/files/centos_6.2_x86_64/output.swirl 
Usage: fingerprint [-q|-c|-y|-d|-i] [options] inputfile1 ... inputfileN

fingerprint: error: The file output.swirl could not be opened on this system.
```
(is the file broken? absent?? what?)

to
```
$> PYTHONPATH=$PWD bin/fingerprint -d ./tests/files/Ubuntu_12.04_x86_64/output.swirl                
Usage: fingerprint [-q|-c|-y|-d|-i] [options] inputfile1 ... inputfileN
 
fingerprint: error: The file output.swirl could not be opened on this system: [Errno 2] No such file or directory: 'output.swirl'.
```
(sounds like an internal issue on assumptions of location of the input file)