remove masking of s7 protocols errors

[hmcguinn]

This MR removes the masking of errors for the siemens s7 protocol scanner.

The current behaviour:

➜  zgrab2 git:(hmcguinn/s7-misidentification) ✗ echo $IP | ./zgrab2 siemens                   
INFO[0000] started grab at 2021-07-20T20:58:16-04:00    
{"ip":"$IP","data":{"siemens":{"status":"success","protocol":"siemens","result":{"is_s7":true},"timestamp":"2021-07-20T20:58:16-04:00"}}}
INFO[0001] finished grab at 2021-07-20T20:58:18-04:00   
{"statuses":{"siemens":{"successes":1,"failures":0}},"start":"2021-07-20T20:58:16-04:00","end":"2021-07-20T20:58:18-04:00","duration":"1.552316484s"}

The scan is marked as a success when no information is returned.

The updated behaviour:

➜  zgrab2 git:(hmcguinn/s7-misidentification) ✗ echo $IP | ./zgrab2 siemens         
INFO[0000] started grab at 2021-07-20T20:44:09-04:00    
{"ip":"$IP","data":{"siemens":{"status":"unknown-error","protocol":"siemens","result":{"is_s7":true},"timestamp":"2021-07-20T20:44:09-04:00","error":"not a S7 packet"}}}

How to Test

echo $IP | ./zgrab2 siemens

Notes & Caveats

The S7 protocol scanner currently masks errors and will silently fail if a host responds with a ISO 8327-1 "Abort" packet. The scan will be incorrectly marked as a false positive.