Raise a useful exception for incomplete formulae

Library/Homebrew/formula.rb

@@ -73,9 +73,11 @@ class Formula
     when @head && ARGV.build_head?        then @head    # --HEAD
     when @devel && ARGV.build_devel?      then @devel   # --devel
     when @bottle && install_bottle?(self) then @bottle  # bottle available
+    when @stable                          then @stable
     when @devel && @stable.nil?           then @devel   # devel-only
     when @head && @stable.nil?            then @head    # head-only
-    else @stable
+    else
+      raise "Formulae require at least a URL"
     end
   end
 

Library/Homebrew/test/test_formula_validation.rb

@@ -77,4 +77,9 @@ class FormulaValidationTests < Test::Unit::TestCase
       end
     end
   end
+
+  def test_empty_formula_invalid
+    e = assert_raises(RuntimeError) { formula {} }
+    assert_equal "Formulae require at least a URL", e.message
+  end
 end