Error handling for individual taps

Possibly would help diagnose: Homebrew/homebrew#13853.

Library/Homebrew/cmd/update.rb

@@ -20,11 +20,16 @@ module Homebrew extend self
     new_files = []
     Dir["Library/Taps/*"].each do |tapd|
       cd tapd do
+        begin
           updater = Updater.new
           updater.pull!
           report.merge!(updater.report) do |key, oldval, newval|
             oldval.concat(newval)
           end
+        rescue
+          tapd =~ %r{^Library/Taps/(\w+)-(\w+)}
+          onoe "Failed to update tap: #$1/#$2"
+        end
       end
     end