Improve Search API Error Handling

[MRichards99]

Since I've emailed Rolf discussing the datasets relationship in the PaNOSC data model, I will keep #319 open. I will close it once I've got a response and we know a way forward with the data model (whether to raise it further or to follow it as it is).

Description

This PR improves the error signaling for the search API. In #319 (comment), there's a request that showed a 400. Since this is an issue with the validation of the data model, this should really be a 500. My changes fix that for Pydantic's ValidationErrors.

I've also made some general improvements - I've added KeyError to the tuple of standard Python exceptions to catch and moved the assignment of a status_code attribute into its own function (to avoid code duplication).

The version bump will be a fix bump.

Testing Instructions

When using a local instance pointing at ISIS dev ICAT, the following request should return a 500 with a more meaningful error message (about the validation error):
http://{{datagateway-api}}/search-api/documents?filter={"limit": 10}&filter={ "include": [ { "relation": "members", "scope": { "where": { "role": { "neq": "principal_experimenter" } }, "include": [ { "relation": "person" } ] } }, { "relation": "datasets" } ], "limit": 1, "skip": 3

• Review code
• Check GitHub Actions build
• If icatdb Generator Script Consistency Test CI job fails, is this because of a deliberate change made to the script to change generated data (which isn't actually a problem) or is here an underlying issue with the changes made?
• Review changes to test coverage
• Does this change mean a new patch, minor or major version should be made? If so, does one of the commit messages feature fix:, feat: or BREAKING CHANGE: so a release is automatically made via GitHub Actions upon merge?

[codecov]

Codecov Report

Merging #343 (1e42981) into master (dd8bc51) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
+ Coverage   94.45%   94.46%   +0.01%     
==========================================
  Files          39       39              
  Lines        3065     3073       +8     
  Branches      306      307       +1     
==========================================
+ Hits         2895     2903       +8     
  Misses        144      144              
  Partials       26       26              

Impacted Files	Coverage Δ
datagateway_api/src/search_api/helpers.py	100.00% <100.00%> (ø)

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