brew search: properly handle 503 errors.

This avoids crashing with an unknown key error, if the GitHub api
response does not contain the ratelimit headers, e.g. when GitHub is
down. It also tries to display the JSON error message in addition to
the HTTP status.

Closes Homebrew/homebrew#48538.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/utils.rb

@@ -531,7 +531,7 @@ module GitHub
   end
 
   def handle_api_error(e)
-    if e.io.meta["x-ratelimit-remaining"].to_i <= 0
+    if e.io.meta.fetch("x-ratelimit-remaining", 1).to_i <= 0
       reset = e.io.meta.fetch("x-ratelimit-reset").to_i
       error = Utils::JSON.load(e.io.read)["message"]
       raise RateLimitExceededError.new(reset, error)
@@ -543,7 +543,8 @@ module GitHub
     when "404"
       raise HTTPNotFoundError, e.message, e.backtrace
     else
-      raise Error, e.message, e.backtrace
+      error = Utils::JSON.load(e.io.read)["message"] rescue nil
+      raise Error, [e.message, error].compact.join("\n"), e.backtrace
     end
   end