fix: change error for invalid feature param in feature RPC

[mvadari]

High Level Overview of Change

This PR changes the error message for invalid values of feature in the feature RPC. Now, if a non-string is passed into the function, it will return invalidParams, instead of badFeature as it did before.

Context of Change

In rippled 2.2.0, a non-admin version of feature was added. This makes the need for proper error handling more important.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

API Impact

• Public API: Breaking change (in general, breaking changes should only impact the next api_version)

While this is technically a breaking change, I consider it more of a bugfix. Only the error message is changing.

Test Plan

Additional unit tests were added.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.4%. Comparing base (2820feb) to head (61e6e85).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5063     +/-   ##
=========================================
- Coverage     71.4%   71.4%   -0.0%     
=========================================
  Files          796     796             
  Lines        67039   67042      +3     
  Branches     10864   10864             
=========================================
+ Hits         47844   47846      +2     
- Misses       19195   19196      +1     

Files	Coverage Δ
src/xrpld/rpc/handlers/Feature1.cpp	94.3% <100.0%> (+0.5%)	⬆️

... and 5 files with indirect coverage changes

[godexsoft]

Looks correct to me 👍

[intelliot]

OK because this is a relatively new RPC, and it is not involved in how integrations do tx processing (where any potential API breakage would be more painful)

[mvadari]

This is ready to merge.