formulary: handle ScriptError in formula

I added a new `FormulaUnreadableError` subclass of
`FormulaUnavailableError` so existing `rescue`s of
`FormulaUnavailableError` handle this as well.

The new subclass will output the name of the formula with the error
(because this isn't always obvious from the original exception message)
followed by the original error message.

Fixes #1927.

Library/Homebrew/exceptions.rb

@@ -131,6 +131,19 @@ class FormulaClassUnavailableError < FormulaUnavailableError
   end
 end
 
+class FormulaUnreadableError < FormulaUnavailableError
+  attr_reader :formula_error
+
+  def initialize(name, error)
+    super(name)
+    @formula_error = error
+  end
+
+  def to_s
+    "#{name}: " + formula_error.to_s
+  end
+end
+
 class TapFormulaAmbiguityError < RuntimeError
   attr_reader :name, :paths, :formulae
 

Library/Homebrew/formulary.rb

@@ -22,7 +22,11 @@ class Formulary
 
     mod = Module.new
     const_set(namespace, mod)
-    mod.module_eval(contents, path)
+    begin
+      mod.module_eval(contents, path)
+    rescue ScriptError => e
+      raise FormulaUnreadableError.new(name, e)
+    end
     class_name = class_s(name)
 
     begin

Library/Homebrew/test/exceptions_test.rb

@@ -56,6 +56,11 @@ class ExceptionsTest < Homebrew::TestCase
       FormulaClassUnavailableError.new("foo", "foo.rb", "Foo", list).to_s
   end
 
+  def test_formula_unreadable_error
+    formula_error = LoadError.new("bar")
+    assert_equal "foo: bar", FormulaUnreadableError.new("foo", formula_error).to_s
+  end
+
   def test_tap_unavailable_error
     assert_equal "No available tap foo.\n", TapUnavailableError.new("foo").to_s
   end