update-report: tweak migration failure message.

Library/Homebrew/cmd/update-report.rb

@@ -283,9 +283,12 @@ module Homebrew
     EOS
   rescue => e
     ofail <<-EOS.undent
-      #{e}
       #{Tty.white}Failed to migrate HOMEBREW_REPOSITORY to #{new_homebrew_repository}!
-      Please comment with your new error in issue or +1 an existing error:
+      The error was:
+        #{e}
+      Please try to resolve this error yourself and then run `brew update` again to
+      complete the migration. If you need help please +1 an existing error or comment
+      with your new error in issue:
         #{Tty.em}https://github.com/Homebrew/brew/issues/987#{Tty.reset}
     EOS
     $stderr.puts e.backtrace