Show tap/path for non-core formulae on build failure

Closes Homebrew/homebrew#16373.
Closes Homebrew/homebrew#20725.

Library/Homebrew/exceptions.rb

@@ -173,6 +173,12 @@ class BuildError < Homebrew::InstallationError
     else
       require 'cmd/--config'
       require 'cmd/--env'
+
+      unless formula.core_formula?
+        ohai "Formula"
+        puts "Tap: #{formula.tap}" unless formula.tap == "mxcl/master" # tap lies about non-tap externals
+        puts "Path: #{formula.path.realpath}"
+      end
       ohai "Configuration"
       Homebrew.dump_build_config
       ohai "ENV"

Library/Homebrew/formula.rb

@@ -495,6 +495,11 @@ class Formula
     end
   end
 
+  # True if this formula is provided by Homebrew itself
+  def core_formula?
+    path.realpath.to_s == Formula.path(name).to_s
+  end
+
   def self.path name
     Pathname.new("#{HOMEBREW_REPOSITORY}/Library/Formula/#{name.downcase}.rb")
   end