[Actors API] update err desc to be more explicit

[cicoyle]

Ran into some errors with using the Actors API & reminders. This PR is a nice to have making it explicit to newbie actor users what the error means.

This PR fixes one of the issues, which I also had locally and confirm resolves the err:

== APP == 2024/10/22 13:54:53 http: superfluous response.WriteHeader call from [github.com/dapr/go-sdk/service/http.(*Server).registerBaseHandler.func6](http://github.com/dapr/go-sdk/service/http.(*Server).registerBaseHandler.func6) (topic.go:214)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.48%. Comparing base (27248ba) to head (d3077c1).
Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
actor/manager/manager.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #643      +/-   ##
==========================================
+ Coverage   58.04%   62.48%   +4.44%     
==========================================
  Files          55       56       +1     
  Lines        3568     4145     +577     
==========================================
+ Hits         2071     2590     +519     
- Misses       1375     1428      +53     
- Partials      122      127       +5     

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[mikeee]

lgtm