Make Find SCP resilient to missing query-retrievel level

[Enet4]

Fixes a server error situation when the FIND SCP service receives a malformed request with a missing Query/Retrieve Level.

Sumary

• if query-retrieve level is missing or could not be matched, send error response
• use getString to retrieve DICOM text value
• clean up code a bit, removing commented out code and unwanted prints

[Enet4]

Tested this with two different SCU tools:

• dicom-findscu from DICOM-rs: dicom-findscu DICOOGLE-STORAGE@archive:1045 -v -q Modality=CR -q QueryRetrieveLevel= results in a response describing the failure:

  Match #0 Response command:
  (0000,0000) CommandGroupLength           UL (1,  4 bytes): 136
  (0000,0002) AffectedSOPClassUID          UI (1, 28 bytes): "1.2.840.10008.5.1.4.1.2.2.1"
  (0000,0100) CommandField                 US (1,  2 bytes): 32800
  (0000,0120) MessageIDBeingRespondedTo    US (1,  2 bytes): 1
  (0000,0800) CommandDataSetType           US (1,  2 bytes): 257
  (0000,0900) Status                       US (1,  2 bytes): 65024
  (0000,0901) OffendingElement             AT (1,  4 bytes): (0008,0052)
  (0000,0902) ErrorComment                 LO (1, 40 bytes): "Query/Retrieve Level invalid or missing"
  2023-08-31T17:07:29.207364Z  WARN dicom_findscu: Operation failed (status code 65024)

• findscu from dcmtk shows the error in a different way, but gracefully exiting all the same

  findscu -aec DICOOGLE-STORAGE -S -v archive 1045 -k Modality=CR
  I: Requesting Association
  I: Association Accepted (Max Send PDV: 17352)
  I: Sending Find Request (MsgID 1)
  I: Request Identifiers:
  I:
  I: # Dicom-Data-Set
  I: # Used TransferSyntax: Little Endian Explicit
  I: (0008,0060) CS [CR]                                     #   2, 1 Modality
  I:
  I: Received Final Find Response (Cancel: MatchingTerminatedDueToCancelRequest)
  I: Releasing Association

Incidents can be seen from the logs, no stack traces so it's not too noisy:

INFO  DicomLog.LogDICOM (LogDICOM.java:71) - [FINDSCU] cfind FINDSCU(4) --  Modality - CR
INFO  net.PDUEncoder (PDUEncoder.java:379) - FINDSCU(4) << 1:C-FIND-RSP[pcid=1, status=fe00H
        error=Query/Retrieve Level invalid or missing
        cuid=1.2.840.10008.5.1.4.1.2.2.1/Study Root Query/Retrieve Information Model - FIND]
INFO  net.Association (Association.java:991) - FINDSCU(4) >> A-RELEASE-RQ
INFO  net.PDUEncoder (PDUEncoder.java:120) - FINDSCU(4) << A-RELEASE-RP
INFO  net.Association (Association.java:916) - FINDSCU(4): close Socket[addr=/...,port=59755,localport=1045]