install: add backtrace on failure

The error message might be useless alone

Library/Homebrew/cmd/install.rb

@@ -267,6 +267,7 @@ module Homebrew
     # Need to rescue before `FormulaUnavailableError` (superclass of this)
     # is handled, as searching for a formula doesn't make sense here (the
     # formula was found, but there's a problem with its implementation).
+    $stderr.puts e.backtrace if Homebrew::EnvConfig.developer?
     ofail e.message
   rescue FormulaUnavailableError => e
     if e.name == "updog"

Library/Homebrew/formulary.rb

@@ -35,6 +35,7 @@ module Formulary
     begin
       mod.module_eval(contents, path)
     rescue NameError, ArgumentError, ScriptError => e
+      $stderr.puts e.backtrace if Homebrew::EnvConfig.developer?
       raise FormulaUnreadableError.new(name, e)
     end
     class_name = class_s(name)