Improve HOMEBREW_BREW_FILE unset failure case.

Print a better exception message and handle this in more cases rather
than producing confusing errors when it’s unset.

Library/Homebrew/brew.rb

+unless ENV["HOMEBREW_BREW_FILE"]
+  raise "HOMEBREW_BREW_FILE was not exported! Please call bin/brew directly!"
+end
+
 std_trap = trap("INT") { exit! 130 } # no backtrace thanks
 
 # check ruby version before requiring any modules.

Library/Homebrew/config.rb

-if ENV["HOMEBREW_BREW_FILE"]
-  # Path to `bin/brew` main executable in {HOMEBREW_PREFIX}
-  HOMEBREW_BREW_FILE = Pathname.new(ENV["HOMEBREW_BREW_FILE"])
-else
-  odie "HOMEBREW_BREW_FILE was not exported! Please call bin/brew directly!"
+unless ENV["HOMEBREW_BREW_FILE"]
+  raise "HOMEBREW_BREW_FILE was not exported! Please call bin/brew directly!"
 end
 
+# Path to `bin/brew` main executable in HOMEBREW_PREFIX
+HOMEBREW_BREW_FILE = Pathname.new(ENV["HOMEBREW_BREW_FILE"])
+
 # Where we link under
 HOMEBREW_PREFIX = Pathname.new(ENV["HOMEBREW_PREFIX"])
 

Library/Homebrew/test/lib/config.rb

+unless ENV["HOMEBREW_BREW_FILE"]
+  raise "HOMEBREW_BREW_FILE was not exported! Please call bin/brew directly!"
+end
+
 require "tmpdir"
 require "pathname"