Context parse error should not return 500 response

[markphelps]

Describe the Bug

When testing via https://try.flipt.io, I came across an issue when trying to evaluate a context that contained a field that was trying to be parsed as a boolean x, which obviously cant be parsed as a bool. This was resulting in a 500 response being return from the API (REST).

The evaluation console also showed an empty response, instead of displaying the actual error.

Version Info

Run flipt --version and paste the output here:

To Reproduce

https://github.com/flipt-io/flipt/releases/tag/v1.11.0

Expected Behavior

• A 4xx response or similar, the server should not return a 500
• In the UI, the evaluation console should display the error (and highlight the window in red) instead of an empty response (in green)

Additional Context

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Examples:

• OS
• Config file used
• Database used (SQLite, MySQL, or Postgres)
• Screenshots
• Exported data from the database

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