Merge pull request #5593 from MikeMcQuaid/missing-env-var-exception

brew.rb: duplicate MissingEnvironmentVariables exception.

Author: MikeMcQuaid

Library/Homebrew/brew.rb

@@ -10,6 +10,9 @@
   raise "Homebrew must be run under Ruby 2.3! You're running #{RUBY_VERSION}."
 end
 
+# Also define here so we can rescue regardless of location.
+class MissingEnvironmentVariables < RuntimeError; end
+
 begin
   require_relative "global"
 rescue MissingEnvironmentVariables => e