Require a formatter in error_formatter and drop the unreachable 406 throw

[ericproulx]

error_formatter :json — with no formatter, positionally or as with: — registered nil:

def error_formatter(format, options = nil, with: nil)
  formatter = with || options   # nil
  inheritable_setting.add_error_formatter(format.to_sym, formatter)
end

Grape::ErrorFormatter.formatter_for hands a registered value back verbatim through its key? short-circuit, so that format resolved to no formatter at all and Middleware::Error#format_message fell through to a throw :error, 406.

That throw could never deliver its 406. Nothing catches :error around the call: #call! has already left its catch by the time error_response runs with the caught value, and run_rescue_handler re-renders outside its own catch. So it raised UncaughtThrowError, render_response rescued it, and the request answered with the failsafe 500 Internal Server Error in text/plain — whatever status and message the API had asked for:

path (format :json + error_formatter :json)	response	format_message calls	uncaught throws
error!('nope', 422)	500 text/plain	2	2
validation failure	500 text/plain	3	2

Both ends are fixed here:

• DSL::RequestResponse#error_formatter raises ArgumentError when no formatter is given, so the mistake surfaces where it is made rather than on the first error the API renders.
• format_message drops the dead branch — formatter_for always resolves to a registered formatter, default_error_formatter or ErrorFormatter::Txt.

An API that passes a formatter is unaffected, and a format with no error_formatter of its own keeps falling back to default_error_formatter and then to Txt (pinned by a new middleware spec).

🤖 Generated with Claude Code

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