Use 'path or URL' as tap for non-tap, non-core formulae

Library/Homebrew/exceptions.rb

@@ -176,7 +176,7 @@ class BuildError < Homebrew::InstallationError
 
       unless formula.core_formula?
         ohai "Formula"
-        puts "Tap: #{formula.tap}" unless formula.tap == "mxcl/master" # tap lies about non-tap externals
+        puts "Tap: #{formula.tap}"
         puts "Path: #{formula.path.realpath}"
       end
       ohai "Configuration"

Library/Homebrew/formula.rb

@@ -489,9 +489,10 @@ class Formula
   def tap
     if path.realpath.to_s =~ %r{#{HOMEBREW_REPOSITORY}/Library/Taps/(\w+)-(\w+)}
       "#$1/#$2"
-    else
-      # remotely installed formula are not mxcl/master but this will do for now
+    elsif core_formula?
       "mxcl/master"
+    else
+      "path or URL"
     end
   end