download_strategy: use (more consistent) TypeError.

Rather than a base `RuntimeError`.

Fixes https://github.com/Homebrew/brew/issues/6839

Library/Homebrew/download_strategy.rb

@@ -1122,7 +1122,7 @@ class DownloadStrategyDetector
     when :post                   then CurlPostDownloadStrategy
     when :fossil                 then FossilDownloadStrategy
     else
-      raise "Unknown download strategy #{symbol} was requested."
+      raise TypeError, "Unknown download strategy #{symbol} was requested."
     end
   end
 end