Avoid exception on adding ip device with wrong ip/hostname

[johannaengland]

Closes #2696.

Doesn't really show a nice error message, only catches the exception.
If I should show a clearer error message I would need some pointers on how to do that, since I don't see a way to send the error message up

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[github-actions]

Test results

     12 files       12 suites   12m 45s ⏱️
3 221 tests 3 221 ✔️ 0 💤 0 ❌
9 138 runs  9 138 ✔️ 0 💤 0 ❌

Results for commit 17ece24.

[lunkwill42]

I think this solves the problem slightly at the wrong level. This will also conflict heavily with the ongoing changes in #2711 (which has shown me that the connectivity tests on the backend are named and structured in stupid ways).

Anyway, I have a working JS suggestion for checking the backend response and distinguishing between internal server errors and bad requests - and I think a request to check the connectivity of an invalid IP address should return a 400 Bad Request.

However, the functionality currently supports both IP addresses and host names as input. If you enter a hostname that resolves to multiple IP addresses in the netbox form, the JS UI will force you to select one of the IP addresses before allowing a connectivity test. If it only resolves to a single address, the hostname is used as input, and that hostname is fed directly to the underlying SNMP library, resulting in a sort of nondescript exception being raised.

A more proper solution should probably figure out the actual IP address at the Django view level, and return a 400 Bad Request with a JSON error payload if the IP addr is invalid. Actual unhandled errors from the SNMP layer should still cause a 500 internal error, but the front end should treat 500 and 400 differently.

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 55.19%. Comparing base (ccb83d1) to head (17ece24).
Report is 553 commits behind head on master.

Files	Patch %	Lines
python/nav/Snmp/pynetsnmp.py	50.00%	2 Missing ⚠️
python/nav/web/seeddb/page/netbox/edit.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2713      +/-   ##
==========================================
- Coverage   55.19%   55.19%   -0.01%     
==========================================
  Files         561      561              
  Lines       40917    40920       +3     
==========================================
+ Hits        22584    22585       +1     
- Misses      18333    18335       +2     

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